Image source, Getty Images

We've spoken to a local worker, who says there was a big police presence when he arrived on the scene.

He says his coworkers, who witnessed the attack, told him "someone threw a Molotov cocktail" at a group of people who were gathered there for a peaceful demonstration calling for the release of Israeli hostages in Gaza, and there were people running in the street.

"When I arrived at my job, there were three girls alone and I was the only man there. They were very scared."

He also explained that the boulevard is usually a very busy area, often hosting events and concerts.

Run for Their Lives was the group holding a march when the attack happened.

The group holds walking and running events around the world calling for the release of Israeli hostages held by Hamas, according to their website.

It says its events are not protests, but “peaceful walks”.

Their website says there are currently 230 active groups around the world, with the majority in North America and Europe.

The groups meet once a week for a 1km walk wearing red T-shirts. They also carry national flags of the citizens who are among the hostages still held in Gaza.

The Run for Their Lives Instagram account has more than 6,000 followers. Their Facebook group has more than 2,000 members.

The movement was started by a group of Israelis in California, but local events are “independently led”, according to their website.

Danny Danon, Israel's ambassador to the UN, has reacted on social media to tonight's attack in Boulder, calling it "terrorism" and calling for "concrete action".

In a post on X, the ambassador says "Jewish protesters were brutally attacked".

"Terrorism against Jews does not stop at the Gaza border – it is already burning the streets of America", he adds.

The FBI has also called it a "targeted terror attack" - but local police say it is too early to say that or speculate on motive.

FBI Deputy Director Dan Bongino has posted online saying they have a leadership team on the ground and will provide an update shortly.

He has reiterated that the FBI is treating what happened in Boulder as "an act of terror", which contradicts the Boulder County Police Department who we heard from earlier.

Bongino says it is being investigated as "an act of ideologically motivated violence" based on the "early information, the evidence, and witness accounts".

Police say there will be a press conference at 18:30 local time (01:30 BST).

We are seeing several images and videos on social media of the reported suspect, seemingly minutes after the incident took place.

While we work on verifying these images, we can describe what we are seeing - some show the apparent suspect, who is shirtless, holding two bottles with pieces of red cloth and a clear liquid inside. As we just reported, the liquid was flammable, according to a source speaking to CBS.

In one particular video, a person recording tells a police officer the man was “throwing Molotov cocktails” - though police earlier said they were unable to confirm this.

Videos also show the man being detained by police at the scene.

We will bring you more information as we have it.

Our news partners CBS have heard from a law enforcement source who says the suspect is in custody.

The source says at least five people have sustained burns, with one or two of those in a serious condition in hospital.

CBS reports that bottles, containing a flammable liquid, were thrown at a group gathered at the Boulder County Historic Courthouse.

People had been taking part in an event aimed at raising awareness of the Israeli hostages still being held by Hamas in Gaza.

Image source, Reuters

The Boulder Police Department says helicopters circling above the scene on Pearl Street are making it "impossible" for their drones to investigate.

The agency has put a plea out on social media calling for all aircraft to avoid airspace over Pearl Street.

Boulder Police Chief Stephen Redfearn also answered some questions from reporters - including how many people were injured and what types of injuries they sustained.

The outdoor mall where the attack took place was crowded with people, including those gathered for a pro-Israeli demonstration, authorities said.

Police are examining whether the demonstration was targeted, but have yet to confirm a link.

The Jewish community in Boulder released a statement condemning the attack, and saying people at the demonstration, which was organised by a group called "Run for Their Lives," were injured.

"Our hearts go out to those who witnessed this horrible attack, and prayers for a speedy recovery to those who were injured," Boulder's Jewish Community said in a joint statement. "When events like this enter our own community, we are shaken."

The Run for Their Lives group, which aims to show solidarity for Israeli hostages in Gaza, meets regularly at the Pearl Street Mall to "share the message far and wide (specifically via social media) that the hostages have not been forgotten" during a "peaceful walk" but not a protest, according to the group.

Colorado's Attorney General Phil Weiser just released a statement calling the attack at the Pearl Street Mall "targeted". The FBI has called it a terror attack - but local police said during their first press conference that it was too early to tell.

“From what we know, this attack appears to be a hate crime given the group that was targeted," Weiser said.

A group of people peacefully demonstrating to show solidarity for Israeli hostages in Gaza were at the Pearl Street Mall this afternoon when the attack unfolded.

“People may have differing views about world events and the Israeli-Hamas conflict, but violence is never the answer to settling differences," Weiser says in the statement. "Hate has no place in Colorado.”

We can now bring you some more from Boulder Police Chief Stephen Redfearn, who says the police are currently interviewing witnesses and asks people to avoid speculation.

Videos of the attack circulating online show a chaotic scene near a fountain at Boulder's outdoor Pearl Street Mall in Colorado.

Smoke can be seen filling the air while people run in several directions. At least two people appear to be helping others who have fallen to the ground.

In the background, part of the grass appears to be on fire, and people can be heard shouting in the background.

A separate video shows police later wheeling people away from the scene on stretchers.

Redfearn says multiple people were injured in the attack but does not exactly how many.

An area hospital received multiple patients, and some were transferred to other hospitals.

Those patients have a range of injuries from "very serious" to more minor.

Officials say there will be another update later tonight.

A reporter asks if police will provide additional security for Jewish and pro-Israeli protesters.

Redfearn says he cannot confirm at this time that this was an attack targeted at a specific group of people, but he understands there are a lot of tensions and issues in the US right now.

He says the police will "react accordingly" once they have a clear motive - including additional security and other measures.

The police chief is continuing to take questions. When someone asks if a Molotov cocktail was thrown in the attack, Redfearn says it's too early to tell, but says the suspect was "setting people on fire".

The suspect is an adult male who they are working to identify, he adds.

A reporter asks if a protest was going on when the attack happened.

Redfearn says the area is a "walking pedestrian mall" and was crowded with visitors. He says a group of pro-Israel people were gathered in the mall and were demonstrating peacefully.

He said they are interviewing witnesses to decipher if the group was targeted.

Redfearn is asked if he would call it a "targeted terror attack" as the FBI has and he says that it is "way too early" to speculate on the motive. He says local police are not yet calling it a targeted terror attack.

Chief Redfearn says the scene of the attack is closed off, including businesses, while investigations are ongoing.

He adds that the area is not safe yet and that officers are investigating a "vehicle of interest".

He says this was a "beautiful Sunday afternoon in downtown Boulder" and this act is "unacceptable".

Dozens of people are working to figure out what happened, he says, and asks for patience and for people to think of the victims and their families.

Boulder Police Chief Stephen Redfearn says several people were injured in the incident, including injuries consistent with burns as well as other injuries.

He says police found the suspect and took him into custody "without incident".

The suspect was also taken to the hospital with minor injuries, the police chief says.

He says that they can't release the suspect's name yet as they investigate the incident.

Boulder Police Chief Stephen Redfearn is speaking and offering what he calls "very preliminary information" that they received calls about an attack at 1:26 pm local time (20:26 BST).

People who called authorities said there was a man with a weapon and people were being set on fire.

Officers responded quickly, he said.

A news conference is now under way in Boulder, Colorado, after police said multiple people were injured after an incident.

We'll bring you the latest updates from officers as we get them.

Stick with us.