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, Calif., that drew thousands of people and led to the peaceful shutdown of the nation's fifth-busiest port. After midnight, the vandalism began.

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.

"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."

The 3,000-person protest outside the port Wednesday 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.

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.

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

Later Thursday, 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 downtown demonstration was marred by an "autonomous" group.

Police in riot gear arrested more than 80 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.

Chris Hedges, who was demonstrating at Goldman Sachs' headquarters in New York, said the Oakland clashes are a reminder that protesters 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.