Authorities said the confrontation erupted on Monday night as police tried to disperse a crowd that was disrupting traffic near the Denver City and County Building, but one protester said police "were coming at us" with no warning.
Police said they could not confirm any arrests, but after the confrontation subsided, about 20 people remained behind in police custody, sitting on a sidewalk as officers took them away one by one.
Earlier, officers led at least two people away as the crowd chanted "Let them go!"
Some of the protesters threw bags containing a coloured liquid at police.
Kaycee Ryann and Eric Finch said they were in the crowd marching through Civic Center Park, which lies between the City-County Building and the state Capitol, when police tried to split the crowd into smaller groups.
"There was no warning. We weren't coming at them. They were coming at us," Finch said.
AP Television News video showed one group of protesters counting down from 10 and then charging at police. They quickly retreated as police shoved them back. Some of the officers gripped their batons, one hand at either end, as they pushed the protesters back.
Finch said he was struck by rubber pellets and a baton.
Polly White of the Joint Information Center, a command set up by city, state and federal authorities to field media inquiries during the convention, said she had no reports of police firing rubber pellets.
Ryann and Finch described themselves as anti-capitalists who were protesting ecological devastation. They said others in the crowd were protesting other issues.
It was believed to be the first time a police-protester confrontation turned physical and the first time officers used any kind of chemical spray since demonstrations began on Sunday, a day before the convention.
Ron Kovic, a paralyzed Vietnam veteran and anti-war activist who led a peaceful march the day before, hurried to the scene in his wheelchair from his downtown hotel after he heard about the confrontation. "We must remain nonviolent. We must have the high moral ground," he told the crowd.
"There's a powerful police presence here. The chill of 1968 is in the air of Denver," said Kovic, whose story was chronicled in the book and movie Born on the Fourth of July.
He was referring to the violence that erupted during the 1968 Democratic National Convention in Chicago when police clashed with anti-war protesters.
