Police in riot gear arrested dozens of protesters in downtown Oakland, where bands of masked protesters took over a vacant building, erected roadblocks and threw chunks of concrete and firebombs. Five people and several officers were injured.

It "has to be nonviolent, or else it will just end. We won't get the support," he said. "It doesn't mean you can't agitate people. But you can't also be breaking windows and burning."

Bob Norkus at the Occupy Boston camp said the riots didn't represent the broader movement and likely wouldn't have a lasting effect on it, either. The movement is still evolving and mistakes are inevitable, he said.

So far, few cities have reached the level of Oakland, a unique place with a long history of tensions between residents and police.

The group released a statement Thursday night saying it doesn't support vandalism but would not take an official position until Friday's night "General Assembly" meeting.

"It is unfortunate that the unprecedented mobilization and engagement of tens of thousands of our neighborhood in this beautiful Oakland city should be marred by broken windows and graffiti," Laura Long said, reading the statement. "Occupy Oakland does not advocate violence and has no interest in supporting actions that endanger the community and possibilities that it has worked to build."

On Thursday afternoon, representatives from the Occupy Oakland media committee read a statement saying participants supported the goal of reclaiming empty buildings to serve the public but regretted that their daylong downtown demonstration was marred by an "autonomous" group.

The street spasm that followed when about 200 people tried to take over a vacant building, however, raised questions about whether a movement with no organizational structure and no high-profile leaders can -- or should -- do anything to stop those they called troublemakers.

The 3,000-person protest outside the port Wednesday night represented an escalation in tactics as demonstrators targeted a major symbol of the nation's commerce with peaceful rallies and sit-ins, managing to effectively suspend maritime operations there for the night.

"They don't speak for the majority of people who were here yesterday," said Hadas Alterman, a college student who was gathering trash at a tent camp near Oakland City Hall. "That was an hour of action, and we were out here for 12 hours and it was peaceful."

Hours after a small group of what city leaders called "provocateurs" clashed with riot police, set fires and shattered windows early Thursday, demonstrators across the country condemned the violence and wondered whether it was a turn that would destroy their cause.

OAKLAND, Calif.—Occupy Wall Street protesters had just half a day to celebrate what they saw as their biggest victory so far: a daylong gathering in Oakland that drew thousands of people and led to the peaceful shutdown of the nation's fifth-busiest port. Then, after midnight, the vandalism began.

Oakland officials said 103 people had been arrested by 3 p.m. Thursday, including two 17-year-olds who were cited and released.

Chris Hedges, who was demonstrating at Goldman Sachs' headquarters in New York, said the clashes in Oakland are a reminder to protesters that they should only respond peacefully to police actions.

"It's awful. But police want people to break windows and set cars on fire, because it's the kind of thing they know how to master -- with force," he said before being led quietly away in handcuffs.

Raymon Curtis, who was protesting in Portland, Ore., said he doesn't believe the police in his city are seeking violence.

"I looked in their eyes and at first I thought it was a hard look," Curtis said. "Then, I realized it was the same look I had when I went to prison for the first time. They're terrified."

Some protesters said violence can bring attention to the cause.

"This thing has to escalate so people see the violence and who is protecting the interests of the corporations," said Denver protester Dwayne Hudson, standing next to a grill with logs burning over charcoal to stay warm after a snowstorm.

The far-flung movement challenging the world's economic systems and distribution of wealth has gained momentum in recent weeks, with Oakland becoming a rallying point after an Iraq War veteran was injured in clashes with police last week.

Organizers called for a general strike on Wednesday, and supporters in New York, Philadelphia, Los Angeles and elsewhere held smaller-scale demonstrations, some in solidarity with their Oakland counterparts.

Protesters in Oakland and in other cities viewed the day's events and the port shutdown as a significant victory.

Police, who had little to no presence during the protest during the day, said about 7,000 people participated in largely peaceful demonstrations. There were a few incidents of vandalism at local banks and businesses.

An accounting of the financial toll from the port shutdown was not immediately available.

A protest organizer in Chicago, Joshua Kaunert, said the shutdown was an "amazing" event for the movement, but he didn't want to speculate on what effect the violence would have. He said the lack of a formal leadership structure -- and the emphasis on what he called a "true, direct democracy" -- makes it difficult to weed out potential troublemakers.

"As a movement, it is definitely hard to keep that kind of element away, but that's a double-edged sword," Kaunert said. "If you want true, direct democracy, you're going to have issues, regardless."

The movement's tent camps in public parks have drawn all types of people, including the homeless, families and anarchists.