Demonstrators started fires near the White HouseImage caption: Demonstrators started fires near the White House
It's not always clear why people who are normally non-violent can endorse protest violence.
But this could in part be explained by moral psychology, says Marloon Moojiman, an assistant professor in organisational behaviour at Houston's Rice University.
A person's sense of morality is central to how they see themselves, so "when we see something as immoral, it creates strong feelings, because we feel our understanding of morality has to be protected".
"This can override other concerns people have about keeping peace... If you think the system is broken, you're going to want to really do something drastic to show that that's not acceptable."
This can apply to a wide range of beliefs - for example, in an extreme case, someone who thinks abortion is a moral outrage may be more likely to say it's OK to bomb an abortion clinic, he says.
Research suggests that social media echo chambers could also make people more susceptible to endorsing violence, if they believe that their peers have the same moral views as them, he adds.
A protester wears a mask during a gathering in Washington StateImage caption: A protester wears a mask during a gathering in Washington State
"We haven’t seen anything quite like this since 1968," says Darnell Hunt, Dean of Social Sciences at UCLA.
The assassination of Martin Luther King that year sparked protests across the US.
Hunt believes the riots are also "an indication of the stress and strain that people feel they're being subjected to".
"We're in the middle of a pandemic - combined with the fact people of colour are disproportionately impacted in the Covid-19 death rate, and unemployment," he said.
"I think all of these things have created this tinderbox that literally exploded in the past few nights."
He believes that the protests go deeper than George Floyd's death in police custody.
"You can't think about police brutality, and the profiling of certain communities, without thinking about the inequalities that exist in society and fuel those concerns."
"The George Floyd case was not the cause - it's more like the straw that broke the camel's back. You could argue even the police killings are symptoms - the underlying cause is white supremacy, racism, and things the US has not fundamentally dealt with."
Dystopian movie or American Spring - world media on US protests
BBC Monitoring
The world through its media
ReutersCopyright: Reuters
The sheer extent and ferocity of the US protests have widely caught the attention of news organisations around the world.
Latin America's media dedicated front-page headlines and photographs to the "fire and fury" of street protests raging in the US over the death of a black man during a Minneapolis police arrest.
Their reports spoke of "chaos", "scenes of barbarism" and anger against the "virus of racism".
In Russia, a five-minute report on state news channel Rossiya featured cases of heavy-handed police tactics, such as a police car driving into a crowd of protesters.
The report concluded that the situation resembled a "dystopian movie".
Pan-Arab TV channels, too, gave rolling coverage to the protests, highlighting the most prominent incidents of violence.
Many social media users condemned the police brutality. Some called the protest movement “the American Spring”.
And some conservative newspapers in Iran referred to the curfews imposed in US cities as "martial law".
Hardline Keyhan's front page headline read "Martial law in 25 American cities", and the banner headline of Vatan-e Emruz newspaper was "US under military boots".
Why do some protests turn violent?
Helier Cheung
BBC News, Washington DC
Getty ImagesCopyright: Getty Images
Tear gas was deployed in Santa Monica, California, on SundayImage caption: Tear gas was deployed in Santa Monica, California, on Sunday
Most of the protests over George Floyd's death began peacefully - and many stayed peaceful. But in some cases, demonstrators set police cars on fire, vandalised property, or looted shops. Why?
Prof Clifford Stott, an expert in crowd behaviour and public order policing at Keele University in the UK, says that "riots are a product of interactions - largely to do with the nature of the way police treat crowds".
"A lot of police forces don't invest in de-escalation training, and rely too heavily on a militarised type of approach, which plays a major role in why otherwise peaceful protests can become violently confrontational."
In that respect, he says, there are some parallels with the 2019 Hong Kong protests where police tactics that were seen as heavy-handed galvanised protesters and made them more confrontational.
China and Iran take aim at US over problems of racism
Getty ImagesCopyright: Getty Images
China and Iran have taken the opportunity to criticise the US over its problems of racism and police violence.
"Black people's lives are also lives. Their human rights must also be guaranteed," Chinese foreign ministry spokesman Zhao Lijian told reporters in Beijing.
"Racism against ethnic minorities in the US is a chronic disease of American society. The current situation reflects once more the severity of the problems of racism and police violence in the US," Zhao added.
Meanwhile, Iran's foreign ministry spokesman Abbas Mousavi spoke briefly in English at his
weekly televised briefing, saying: "To the American officials and
police: stop violence against your people and let them
breathe!"
In pictures: Washington DC hit by violent protests
Getty ImagesCopyright: Getty Images
Washington DC has seen some of the worst violence overnight, with protesters setting up burning barricades and throwing stones at riot police deployed in the US capital.
Cars were torched, and properties looted and vandalised.
There has been widespread anger over some of the tactics employed by police throughout the demonstrations.
There have also been moments where police have stood together with peaceful protesters, enraged by the death of George Floyd.
Here are some of the standout moments:
A video posted to Twitter showed law enforcement in Minneapolis shooting paint rounds at residents while they were implementing a curfew. One shouts: "light 'em up".
In New York, a police car was seen ramming into a group of protesters. Mayor Bill de Blasio - whose daughter was arrested during separate demonstrations - said the incident was under investigation, but added that the officers may have had no other option.
Man dies as shots exchanged during protest in Kentucky
A man has died in Louisville, Kentucky, early on Monday after police, the National Guard and protesters exchanged gunfire, local officials are quoted as saying.
It happened as police and the National Guard were trying to disperse crowds.
Louisville Metro Police Department (LMPD) chief Steve Conrad told NBC's local affiliate WAVE that one person was confirmed dead at the scene. No further details were immediately available.
Last week, a 19-year-old man was shot and killed during protests in Detroit, Michigan.
That shooting did not involve police, US media quoted officers as saying.
Why was Trump taken into a bunker?
ReutersCopyright: Reuters
Protesters gathered in Lafayette Park near the White HouseImage caption: Protesters gathered in Lafayette Park near the White House
It has emerged that in Friday night's unrest, President Donald Trump was briefly taken by the secret service into an underground bunker at the White House.
Washington DC has seen some of the worst violence, with buildings not far from the White House vandalised, cars set on fire, and a historic church attacked.
In tweets, Mr Trump has described the riots as domestic terrorism, blaming left-wing groups and anarchists for the violence.
But some Republicans have expressed concern that his combative approach may be fuelling the unrest.
Democratic presidential candidate Joe Biden - who has visited one protest site in Wilmington, Delaware, and has promised to listen to the concerns of protesters - has urged demonstrators to remain calm.
George Floyd's brother condemns violence and calls for peace
EPACopyright: EPA
George Floyd's younger brother, Terrence, has condemned violent protests continuing across the US, stressing that George stood for peace.
"I'm outraged, too. Sometimes I get angry. I want to go crazy. My brother wasn't about that. You'll hear a lot of people saying, 'He was a gentle giant'," Terrence told ABC News.
He said he would go later on Monday to the site in Minneapolis where his brother was last seen alive to "let people know, just channel your anger elsewhere".
Terrence said he wanted to see charges of first-degree murder brought against the police officer seen kneeling on his brother's neck. Other officers who were at the scene should be charged as well, he said.
YouTuber Jake Paul denies looting Arizona mall
Getty ImagesCopyright: Getty Images
YouTuber Jake Paul has denied taking part in looting following a protest against racial inequality in Scottsdale, Arizona.
The 23-year-old says he was only there to film content for his 20 million YouTube followers.
"Neither I nor anyone in our group was involved in any looting or vandalism," he wrote on Twitter and Instagram.
Twelve people were arrested at Scottsdale Fashion Square after "millions of dollars of damages and theft" took place inside shops.
"We filmed everything we saw in an effort to share our experience and bring more attention to the anger felt in every neighbourhood we travelled through," Paul wrote.
Demonstrations have taken place near the White HouseImage caption: Demonstrations have taken place near the White House
Welcome to our live coverage of the US unrest over the death in police custody of African-American George Floyd. We will be keeping you updated throughout the day with the latest developments.
Here are some of the main headlines coming out of the US overnight:
Riot police clashed with protesters in New York, Chicago, Philadelphia and Los Angeles, firing tear gas and pepper bullets
The National Guard - the US reserve military force for domestic emergencies - said that 5,000 of its personnel had been activated in 15 states and Washington, DC
Curfews have been imposed in nearly 40 cities, but protesters have largely ignored them
President Donald Trump was briefly taken by the secret service into an underground bunker at the White House on Friday, for his safety
Police in Washington DC have fired tear gas at demonstrators who set fire to properties near the White House. They include a historic church, St John's Episcopal Church, known as the church of the presidents, near the White House
At least 4,400 people have been arrested over several days of protests, according to a tally compiled by the Associated Press news agency
Live Reporting
Edited by Paulin Kola
All times stated are UK
Get involved
What makes people think violence is OK?
Helier Cheung
BBC News, Washington DC
It's not always clear why people who are normally non-violent can endorse protest violence.
But this could in part be explained by moral psychology, says Marloon Moojiman, an assistant professor in organisational behaviour at Houston's Rice University.
A person's sense of morality is central to how they see themselves, so "when we see something as immoral, it creates strong feelings, because we feel our understanding of morality has to be protected".
"This can override other concerns people have about keeping peace... If you think the system is broken, you're going to want to really do something drastic to show that that's not acceptable."
This can apply to a wide range of beliefs - for example, in an extreme case, someone who thinks abortion is a moral outrage may be more likely to say it's OK to bomb an abortion clinic, he says.
Research suggests that social media echo chambers could also make people more susceptible to endorsing violence, if they believe that their peers have the same moral views as them, he adds.
Read more here.
Could pandemic have exacerbated the anger?
Helier Cheung
BBC News, Washington DC
"We haven’t seen anything quite like this since 1968," says Darnell Hunt, Dean of Social Sciences at UCLA.
The assassination of Martin Luther King that year sparked protests across the US.
Hunt believes the riots are also "an indication of the stress and strain that people feel they're being subjected to".
"We're in the middle of a pandemic - combined with the fact people of colour are disproportionately impacted in the Covid-19 death rate, and unemployment," he said.
"I think all of these things have created this tinderbox that literally exploded in the past few nights."
He believes that the protests go deeper than George Floyd's death in police custody.
"You can't think about police brutality, and the profiling of certain communities, without thinking about the inequalities that exist in society and fuel those concerns."
"The George Floyd case was not the cause - it's more like the straw that broke the camel's back. You could argue even the police killings are symptoms - the underlying cause is white supremacy, racism, and things the US has not fundamentally dealt with."
Dystopian movie or American Spring - world media on US protests
BBC Monitoring
The world through its media
The sheer extent and ferocity of the US protests have widely caught the attention of news organisations around the world.
Latin America's media dedicated front-page headlines and photographs to the "fire and fury" of street protests raging in the US over the death of a black man during a Minneapolis police arrest.
Their reports spoke of "chaos", "scenes of barbarism" and anger against the "virus of racism".
In Russia, a five-minute report on state news channel Rossiya featured cases of heavy-handed police tactics, such as a police car driving into a crowd of protesters.
The report concluded that the situation resembled a "dystopian movie".
Pan-Arab TV channels, too, gave rolling coverage to the protests, highlighting the most prominent incidents of violence.
Many social media users condemned the police brutality. Some called the protest movement “the American Spring”.
And some conservative newspapers in Iran referred to the curfews imposed in US cities as "martial law".
Hardline Keyhan's front page headline read "Martial law in 25 American cities", and the banner headline of Vatan-e Emruz newspaper was "US under military boots".
Why do some protests turn violent?
Helier Cheung
BBC News, Washington DC
Most of the protests over George Floyd's death began peacefully - and many stayed peaceful. But in some cases, demonstrators set police cars on fire, vandalised property, or looted shops. Why?
Prof Clifford Stott, an expert in crowd behaviour and public order policing at Keele University in the UK, says that "riots are a product of interactions - largely to do with the nature of the way police treat crowds".
"A lot of police forces don't invest in de-escalation training, and rely too heavily on a militarised type of approach, which plays a major role in why otherwise peaceful protests can become violently confrontational."
In that respect, he says, there are some parallels with the 2019 Hong Kong protests where police tactics that were seen as heavy-handed galvanised protesters and made them more confrontational.
Read more here.
China and Iran take aim at US over problems of racism
China and Iran have taken the opportunity to criticise the US over its problems of racism and police violence.
"Black people's lives are also lives. Their human rights must also be guaranteed," Chinese foreign ministry spokesman Zhao Lijian told reporters in Beijing.
"Racism against ethnic minorities in the US is a chronic disease of American society. The current situation reflects once more the severity of the problems of racism and police violence in the US," Zhao added.
Meanwhile, Iran's foreign ministry spokesman Abbas Mousavi spoke briefly in English at his weekly televised briefing, saying: "To the American officials and police: stop violence against your people and let them breathe!"
In pictures: Washington DC hit by violent protests
Washington DC has seen some of the worst violence overnight, with protesters setting up burning barricades and throwing stones at riot police deployed in the US capital.
Cars were torched, and properties looted and vandalised.
A number of arrests were made across the city.
Here's our photo gallery reflecting the dramatic events of last night.
Policing put under spotlight amid unrest
There has been widespread anger over some of the tactics employed by police throughout the demonstrations.
There have also been moments where police have stood together with peaceful protesters, enraged by the death of George Floyd.
Here are some of the standout moments:
A video posted to Twitter showed law enforcement in Minneapolis shooting paint rounds at residents while they were implementing a curfew. One shouts: "light 'em up".
In New York, a police car was seen ramming into a group of protesters. Mayor Bill de Blasio - whose daughter was arrested during separate demonstrations - said the incident was under investigation, but added that the officers may have had no other option.
But some police have acted in solidarity with demonstrators.
In Flint, Michigan, Genesee County Sheriff Chris Swanson decided to march alongside protesters in a show of solidarity.
And police in New Jersey have done the same...
Man dies as shots exchanged during protest in Kentucky
A man has died in Louisville, Kentucky, early on Monday after police, the National Guard and protesters exchanged gunfire, local officials are quoted as saying.
It happened as police and the National Guard were trying to disperse crowds.
Louisville Metro Police Department (LMPD) chief Steve Conrad told NBC's local affiliate WAVE that one person was confirmed dead at the scene. No further details were immediately available.
Last week, a 19-year-old man was shot and killed during protests in Detroit, Michigan.
That shooting did not involve police, US media quoted officers as saying.
Why was Trump taken into a bunker?
It has emerged that in Friday night's unrest, President Donald Trump was briefly taken by the secret service into an underground bunker at the White House.
Washington DC has seen some of the worst violence, with buildings not far from the White House vandalised, cars set on fire, and a historic church attacked.
In tweets, Mr Trump has described the riots as domestic terrorism, blaming left-wing groups and anarchists for the violence.
But some Republicans have expressed concern that his combative approach may be fuelling the unrest.
Democratic presidential candidate Joe Biden - who has visited one protest site in Wilmington, Delaware, and has promised to listen to the concerns of protesters - has urged demonstrators to remain calm.
George Floyd's brother condemns violence and calls for peace
George Floyd's younger brother, Terrence, has condemned violent protests continuing across the US, stressing that George stood for peace.
"I'm outraged, too. Sometimes I get angry. I want to go crazy. My brother wasn't about that. You'll hear a lot of people saying, 'He was a gentle giant'," Terrence told ABC News.
He said he would go later on Monday to the site in Minneapolis where his brother was last seen alive to "let people know, just channel your anger elsewhere".
Terrence said he wanted to see charges of first-degree murder brought against the police officer seen kneeling on his brother's neck. Other officers who were at the scene should be charged as well, he said.
YouTuber Jake Paul denies looting Arizona mall
YouTuber Jake Paul has denied taking part in looting following a protest against racial inequality in Scottsdale, Arizona.
The 23-year-old says he was only there to film content for his 20 million YouTube followers.
Photos and videos seen on social media appear to show Paul, whose estimated wealth is more than $11m (£8.8m), inside the Arizona shopping centre as it is being looted.
"Neither I nor anyone in our group was involved in any looting or vandalism," he wrote on Twitter and Instagram.
Twelve people were arrested at Scottsdale Fashion Square after "millions of dollars of damages and theft" took place inside shops.
"We filmed everything we saw in an effort to share our experience and bring more attention to the anger felt in every neighbourhood we travelled through," Paul wrote.
See here for more on the story from Newsbeat.
Thanks for joining us...
Welcome to our live coverage of the US unrest over the death in police custody of African-American George Floyd. We will be keeping you updated throughout the day with the latest developments.
Here are some of the main headlines coming out of the US overnight: