Four women and four men between 52 and 88 years old were taken to hospital oon Sunday, Boulder police said.
Authorities had earlier put the count of the injured at six and FBI special agent in charge of the Denver Field Office, Mark Michalek, said at least one of them was in a critical condition.
"As a result of these preliminary facts, it is clear that this is a targeted act of violence and the FBI is investigating this as an act of terrorism," Michalek said.
He named the suspect as Mohamed Soliman, 45, who was also hospitalised..
FBI Director Kash Patel also described the incident as a "targeted terror attack", and Colorado Attorney-General Phil Weiser said it appeared to be "a hate crime given the group that was targeted".
Boulder Police Chief Stephen Redfearn said he did not believe anyone else was involved.
"We're fairly confident we have the lone suspect in custody," he said.
The attack took place on the Pearl Street Mall, a popular pedestrian shopping district in the shadow of the University of Colorado, during an event organised by Run for Their Lives, an organisation devoted to drawing attention to the hostages seized in the aftermath of Hamas's 2023 attack on Israel.
Israeli Foreign Minister Gideon Saar said on X he was shocked by the "terrible antisemitic terror attack," describing it as "pure antisemitism".
His administration has detained protesters of the war without charge and cut off funding to elite US universities that have permitted such demonstrations.
The incident comes amid heightened tensions in the US over Israel's war in Gaza, which has spurred both an increase in anti-Semitic hate crime as well as moves by conservative supporters of Israel led by President Donald Trump to brand pro-Palestinian protests as anti-Semitic.
In a post to X, a social network, Trump's deputy chief of staff Stephen Miller said Soliman had overstayed his visa.
It was further evidence of the need to "fully reverse" what he described as "suicidal migration", he said.
Brooke Coffman, a 19-year-old at the University of Colorado who witnessed the Boulder incident, said she saw four women lying or sitting on the ground with burns on their legs.
One of them appeared to have been badly burned on most of her body and had been wrapped in a flag by someone, she said.
She described seeing a man standing in the courtyard shirtless, holding a glass bottle of clear liquid and shouting.
"Everybody is yelling, 'get water, get water,'" Coffman said.
Senate Minority Leader Chuck Schumer, a prominent Jewish Democrat, said it was an anti-Semitic attack.
"This is horrifying, and this cannot continue. We must stand up to antisemitism," he said on X.
The attack follows last month's arrest of a Chicago-born man in the fatal shooting of two Israeli embassy employees in Washington, and someone opened fire on a group of people leaving an event hosted by the American Jewish Committee.
The shooting fuelled polarisation in the US over the war in Gaza between supporters of Israel and pro-Palestinian demonstrators.
with reuters
