Conflict coloured the first day of Olympic competition on Saturday after a protest rally erupted into violence in downtown Vancouver, spurring a lengthy showdown between participants and police.

More than 100 protesters, many believed to be anarchists clad in all-black attire with bandanas over their faces, spray painted cars and threw newspaper boxes into windows near the intersection of Granville and Georgia Streets shortly before 10 a.m.

Another 200 protesters remained peaceful, police said later.

The violent faction then marched west, walking backwards in formation holding a ladder as a barricade. Protesters beat drums and chanted slogans such as "Homes not games," and "No Olympics on stolen native land."

Simon Fraser University student Nekita Garcias said she and a friend watched as the protest turned ugly.

"We were right there when the violence started," Garcias said. "There were a lot of parents with kids around, they rushed them away."

The crowd was larger before the violence broke out, Garcias said, but many participants left once the property damage began.

"Some people just walked away once it got violent, they didn't want any part of it."

The VPD Crowd Control Unit and the 2010 Integrated Security Unit followed the crowd west to the entrance of the Stanley Park causeway, where they circled and advanced on the protesters, beating their shields with batons and pushing protesters to the ground in front of them.

Two lines of police trapped protesters between them and began to make arrests.

After several hours, police had protesters corralled in a downtown street and agreed to escort them for a few blocks in exchange for a commitment for the protesters to disperse. A cheer went up in the crowd at the agreement.

Alissa Westergard-Thorpe, of the Olympic Resistance Network, disavowed her group from the vandalism and violence, saying that the protest was "a day of autonomous actions, independent groups, with a diversity of tactics around the Olympics."

She said there was some property damage, but that the people who committed the damage were not the ones arrested. She said she considers the action non-violent as long as no one was hurt, no matter how much damage is caused.

"The people who were arrested were simply marching down the street trying to assert their Charter rights with the police. The arrests were unnecessarily violent."

Westergard-Thorpe did not condemn the broken windows and vandalism that touched off the stand-off.

"People can choose the tactics that they like," she said, saying police were responsible for most of the violence.

There were reports quickly on social media sites that protesters rolled marbles onto the street to disrupt mounted patrols and also threw acid at police, but officials were not immediately able to comment on the reports that surfaced on Twitter.

Protesters forced the torch relay to reroute in the city Friday and have vowed mass gatherings to oppose the Games.

On Friday, the day the Games began, more than 1,500 marchers opposed to a variety of things, demonstrated as the torch run ended and the opening ceremonies were to begin.

When they got near the stadium where the ceremonies were being held, a three-deep line of officers held them back.

Still, the protest was largely peaceful, with only one arrest after two officers were hurt during a confrontation.

Westergard-Thorpe said there are more demonstrations planned in the coming days.

"I think it's really up to police the type of actual violence you're going to see. The actual violence, the only violence toward human beings is coming from the police."

With files from The Canadian Press