Police in riot gear clashed with about 300 protesters about a mile from the site of the Democratic National Convention on Monday night, and some of the demonstrators were hit with pepper spray.
Authorities said the confrontation erupted 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.
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.
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 colored 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.
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 lead 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."
'Freedom Cage' protest
Earlier, a small group of protesters marched to the demonstration zone outside the convention, complaining they were being treated like political prisoners.
Members of the protest group Recreate 68 Alliance visited the fenced-off zone for the first time on Monday and vowed not to return because they oppose the limits on where they can demonstrate.
Protesters derisively call the 47,000-square-foot zone the "Freedom Cage." It's separated from the parking lot around the convention hall by metal fences atop concrete barriers, about 700 feet from the Pepsi Center, where the delegates start gathering Monday night.
"We're being treated by the city of Denver and the Secret Service like political prisoners, like pariahs," said Recreate 68 organizer Mark Cohen.
Cohen and his wife, Barbara, each wore a red inverted triangle similar to the type political prisoners in Nazi Germany were forced to wear.
"We're going to stay here for just a couple of minutes to state our disgust with this abomination, the way the city and Secret Service are tearing the Constitution of the United States to shreds and then we will leave," Mark Cohen said.
Another protester, Holly Heiman, 40, of Green Mountain Falls, said she wanted to show her opposition to what she believes is an oppressive government that won't change no matter who is elected.
Elsewhere, about 100 people rallied at Civic Center Park in downtown Denver to draw attention to people they consider to be political prisoners in the U.S., including American Indian activist Leonard Peltier, who's serving a life sentence for killing two FBI agents during a 1975 standoff on the Pine Ridge Indian Reservation in South Dakota.