OAKLAND, Calif. — Occupy Wall Street protesters had just a few hours to celebrate what they saw as their biggest victory so far: the peaceful shutdown of the nation’s fifth-busiest port. Then the rioting began.

A day after some protesters clashed with riot police, set fires and shattered windows in Oakland, Calif., 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 protest outside the port, which reopened Thursday, represented an escalation in tactics as demonstrators targeted a major symbol of the nation’s commerce with peaceful rallies and sit-ins.

The violence that followed, however, raised questions about whether a movement with no organizational structure and no high-profile leaders can do anything to stop those they called troublemakers.

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.

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.

Chris Hedges, who 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.

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.

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.

A protest organizer in Chicago, Joshua Kaunert, said the shutdown was an “amazing” event for the movement, but 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.”

At the Oakland encampment, Hale Nicholson, who described himself and others as pacifists, said he participated in Wednesday’s march to the port and then went to sleep at the camp around 9:30 p.m. Around 1 a.m., he was awakened by the sound of flash-bangs.

A group of protesters broke into the former Travelers Aid building in order to, as some shouting protesters put it, “reclaim the building for the people.” They voiced anger over budget cuts that forced the closure of a homeless aid program.

They blocked off a street with wood, metal Dumpsters and other large trash bins, sparking bonfires that leapt as high as 15 feet in the air. Several businesses were heavily vandalized. Dozens of protesters wielding shields were surrounded and arrested. Police said they used tear gas and beanbags to disperse the crowd.

It is the kind of posture that Oakland is familiar with, with clashes erupting during the 1960s-era protests over the Vietnam War and the draft, among other issues.

Then, as now, police blamed the violence on a small group of anarchists, many from outside the city. People who participated in the daytime protests returned to help sweep up debris and scrub off graffiti.