The attack unfolded during a weekly scheduled demonstration put on by Run for Their Lives, a pro-Israeli group that holds walks in the outdoor pedestrian mall in solidarity with Israeli hostages in Gaza.

Police got calls around 13:26 local time (20:26 BST) about a man with a weapon and people being set on fire, Boulder Police Chief Stephen Redfearn said at a news conference.

Officers who responded found multiple people injured, including those with burns.

Witnesses told authorities that the suspect used a "makeshift flamethrower and threw an incendiary device into the crowd," said Mark Michalek, who heads the FBI's Denver office.

Redfearn added those devices included Molotov cocktails being tossed at the crowd.

Michalek identified the suspect as Mohamed Sabry Soliman, 45.

Soliman is an Egyptian national, government officials told us.

In 2022, Soliman arrived in California on a non-immigrant visa that expired in February 2023, multiple sources have told CBS News. He had been living in Colorado Springs.

Footage that appeared to be from the attack showed a chaotic scene: smoke filling the air, people running in multiple directions, spots of grass on fire and people injured on the ground.

In images and videos posted online, but not yet verified by the UKWELITIMES, a man who appears to be the suspect is seen without a shirt and holding bottles with liquid with a piece of red cloth inside. He can be heard yelling at the crowd and appears to be advancing on them as some rush to flee.

As he screams, one woman is on the ground and appears injured. People surround her and one man pours water on her body.

Footage shows police rushing to the scene and arresting the suspect. Police say he was taken to the hospital with injuries.

"It is clear that this is a targeted act of violence and the FBI is investigating this as an act of terrorism," Mr Michalek said. "Sadly, attacks like this are becoming too common across the country."

The attack is the second high-profile act of violence in the US in the last two weeks related to the conflict in Gaza.

A man who shouted "free Palestine" fatally shot two Israeli embassy employees outside a Jewish museum in Washington DC on 22 May. The incident happened at a networking event organised by a Jewish organisation.

Colorado's Attorney General Phil Weiser said that from what officials know the attack "appears to be hate crime given the group that was targeted".

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

Israel's foreign minister, Gideon Sa'ar, said he was "shocked" by the incident and called the attack "pure antisemitism".

"Shocked by the terrible antisemitic terror attack targeting Jews in Boulder, Colorado," he wrote on X. "This is pure antisemitism, fuelled by the blood libels spread in the media."

Danny Danon, Israel's ambassador to the UN, similarly was saddened over the attack, calling it "terrorism" and asking for "concrete action" in response.

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

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