Hillary Clinton has issued a short statement on Twitter:

Donald Trump has issued a statement about the attack, after earlier cancelling a campaign trip to Miami. Here's an excerpt:

"Last night’s horrific execution-style shootings of 12 Dallas law enforcement officers – five of whom were killed and seven wounded- is an attack on our country. It is a coordinated, premeditated assault on the men and women who keep us safe.

"We must restore law and order. We must restore the confidence of our people to be safe and secure in their homes and on the street."

You can read the full statement here, external.  

Amateur footage has emerged showing the moment the attackers opened fire at police officers on Main Street in downtown Dallas.

In Washington, the US Capitol building is on lockdown as police search for an individual. This is from the Associated Press:

"The Senate sergeant at arms issued a lockdown alert for the building and visitor center, saying police activity has been reported. 

"Capitol office buildings are open. Capitol police say they are searching for an individual but offered no details. 

"House press gallery staff says the building is on lockdown but that the House met briefly and immediately recessed. Voting scheduled to begin at 9:15 a.m. is postponed, gallery staff said."

Most of the officers injured in the shooting have been released from hospital, police chief David Brown told reporters.

He did not reveal how many, or go into any detail, but he said some would need to return for follow-up treatment.

Seven officers were injured in the attack.

Prayers will be said for the victims and their families at midday local time, in Dallas's Thanksgiving Square, mayor Mike Rawlings has announced.  

• The suspect involved in a stand-off with police said he wanted to kill white people, especially police officers
• He said he was upset about recent police shootings of black men
•   Police negotiated with the man for several hours and exchanged gunfire with him  
• The suspect was killed by an explosive device sent in remotely using a police robot
• Police will not identify the suspects or give any details about them while the investigation is ongoing  
• Police chief David Brown said "divisiveness between our police and our citizens" must stop

The World at One
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Will Rilley works in downtown Dallas at the Crown Plaza hotel, where some of his colleagues could see what was happening from the roof. 

"If innocent lives are being taken, the correct answer is not to take more innocent lives," he told BBC presenter Mark Mardell.  

Donald Trump has cancelled a campaign trip to Miami, his team said in a statement.

The statement said: "The Trump Campaign has canceled the trip to Miami, Florida today, due to the tragic events in Dallas last night. A statement from Mr. Trump will soon follow."

Hillary Clinton earlier said she had cancelled a campaign event in Scranton, Pennsylvania.    

Police chief David Brown said there may be other people involved in the attacks, beyond the four who have been arrested or killed.

"If there is anyone out there that is involved in this, we will find you and we will prosecute you and we will being you to justice," he said.

Dallas mayor Mike Rawlings told the news conference that police will not go into detail about the four suspects while a criminal investigation is under way.

Three of the suspects are in custody and one has died. Both the mayor and police chief David Brown confined their remarks to talking about how events unfolded, and to paying tribute to Dallas law enforcement officers.

Police chief David Brown said the suspect involved in a stand-off in the parking garage did not take his own life, contrary to earlier reports in the media.

He was killed by an explosive device sent in by police attached to a robot, Mr Brown said.

Police chief David Brown also said: "Our profession is hurting, Dallas officers are hurting. We are heartbroken. 

"There are no words to describe the atrocity that occurred to our city. 

"All I know is that this must stop, this divisiveness between our police and our citizens.

"We don't feel much support most days. Let's not make today most days."

The suspect involved in a stand-off with police told officers he was "upset at white people" and "stated he wanted to kill white people, especially white officers", police chief David Brown has just told a news conference.

Mr Brown said: "[The suspect] said he was upset about the black lives matter issue, and was upset about recent police shootings."

He added: "The suspect said he was not affiliated with any groups, he stated that he did this alone."

The Dallas police chief is speaking now:

Presumptive Democratic presidential candidate Hillary Clinton has cancelled a campaign event with Joe Biden in Scranton, Pennsylvania.

Her team said in a statement: "Due to the tragic events in Dallas, the July 8 Hillary for America campaign event with Vice President Biden will be postponed.  

"Details on rescheduling the event will be provided as soon as possible."

Republican congressman Pete Sessions, whose district includes parts of Dallas, has been tweeting his response to the shootings.

The suspect involved in a stand-off with police died from a gunshot wound, shortly after a police explosive device was detonated nearby, mayor Mike Rawlings has just told CNN.

The explosive device was sent in by a robot, he said.

Mr Rawlings said it was still not clear if the suspect took his own life. He was being "very bellicose and saying he was going to take everyone out," the mayor said.

He said the three suspects in custody were "not being real co-operative". The police have "brought in a lot of people" to try to get to the bottom of the motive, he added.

He declined to comment on the races of the suspect involved in the stand-off or the police officers who were killed. 

He said the female suspect - one of three in custody - was a "light skinned African American".

UK Prime Minister David Cameron is "shocked and horrified" by the shootings in Dallas, his spokeswoman has said.

Mr Cameron is expected to discuss the attack with US President Barack Obama on the margins of the Nato summit which both are attending in Poland. 

His official spokeswoman told a regular Westminster media briefing: "The Prime Minister is shocked and horrified by the terrible scenes we have seen overnight.

"He is due to be at the Warsaw summit and I think he will have the opportunity to talk to President Obama about it directly. 

"I think he would echo what the president has said, which is that these attacks on police officers simply doing their job and trying to keep people safe are horrific and cannot be justified."