OAKLAND, Calif. (AP) -- A protest that shut down the Port of Oakland to show the broadening reach of the Occupy Wall Street movement ended in violence when police in riot gear arrested dozens of protesters overnight who broke into a vacant building, shattered downtown windows, sprayed graffiti and set blazes along the way.

At least four protesters were hospitalized Thursday with various injuries, including one needing stitches after fighting with an officer, police said. Several officers were also injured but didn't need hospitalization.

"We go from having a peaceful movement to now just chaos," protester Monique Agnew, 40, said early Thursday.

Protesters also threw concrete chunks, metal pipes, lit roman candles and molotov cocktails, police said.

The far-flung movement of protesters challenging the world's economic systems and distribution of wealth has gained momentum in recent weeks, capturing the world's attention by shutting down one of the nation's busiest shipping ports toward the end of a daylong "general strike" that prompted solidarity rallies across the U.S.

Several thousands of people converged on the Port of Oakland, the nation's fifth-busiest harbor, in a nearly five-hour protest Wednesday, swarming the area and blocking exits and streets with illegally parked vehicles and hastily erected, chain-link fences afterward.

The Port of Oakland reopened after Wall Street protesters removed a blockade. Port officials say workers are returning to their jobs and operations have partially resumed.

Supporters in New York, Philadelphia, Los Angeles and elsewhere staged smaller-scale demonstrations. Each group said its protest was a show of support for the Oakland movement, which became a rallying point when an Iraq War veteran was seriously injured in a clash with police last week.

The larger Occupy movement has yet to coalesce into an organized association and until the port shut down had largely been limited scattershot marches, rallies and tent encampments since it began in September.

Organizers in Oakland viewed the strike and port shutdown as a significant victory. Police said that about 7,000 people participated in demonstrations throughout the day that were peaceful except for a few incidents of vandalism at local banks and businesses.

Boots Riley, a protest organizer, touted the day as a success, saying "we put together an ideological principle that the mainstream media wouldn't talk about two months ago."

His comments came before 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."

Riley, whose anti-capitalist views are well documented, considered the port shutdown particularly significant for organizers who targeted it in an effort to stop the "flow of capital."

The port sends goods primarily to Asia, including wine as well as rice, fruits and nuts, and handles imported electronics, apparel and manufacturing equipment, mostly from Asia, as well as cars and parts from Toyota, Honda, Nissan and Hyundai.

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

The potential for the chaos that ultimately erupted was not something Riley wanted to even consider.

"If they do that after all this ..." said Riley pausing cautiously, then adding, "They're smarter than that."

But the peace that abided throughout a sunny warm autumn Wednesday, as protesters hung a large black banner downtown that read: "DEATH TO CAPITALISM," did not last as a cool midnight approached.

Occupy protesters voicing anger over a budget trim that forced the closure of a homeless aid program converged on the empty building where it had been housed just outside of downtown.

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.

City officials later released a statement describing the spasm of unrest.

"Oakland Police responded to a late night call that protesters had broken into and occupied a downtown building and set several simultaneous fires," the statement read. "The protesters began hurling rocks, explosives, bottles, and flaming objects at responding officers."

Several businesses were heavily vandalized. Dozens of protesters wielding shields were surrounded and arrested.

Protesters ran from several rounds of tear gas and bright flashes and deafening pops that some thought were caused by "flash bang" grenades. Fire crews arrived and suppressed the protesters' flames.

Protesters and police faced off in an uneasy standoff until the wee hours of the morning.

In Philadelphia, protesters were arrested earlier Wednesday as they held a sit-in at the headquarters of cable giant Comcast.

In New York, about 100 military veterans marched in uniform and stopped in front of the New York Stock Exchange, standing in loose formation as police officers on scooters separated them from the entrance. On the other side was a lineup of NYPD horses carrying officers with nightsticks.

"We are marching to express support for our brother, (Iraq war veteran) Scott Olsen, who was injured in Oakland," said Jerry Bordeleau, a former Army specialist who served in Iraq through 2009.

The veterans were also angry that returned from war to find few job prospects.

"Wall Street corporations have played a big role in the wars in Iraq and Afghanistan," said Bordeleau, now a college student. He said private contractors have reaped big profits in those countries.

A New York Post editorial on Thursday called on protesters camped out in Manhattan to leave or have police evict them. "What began as a credible protest against bank bailouts, crony capitalism and the like has, in large measure, been hijacked by crazies and criminals," it said.

In Boston, college students and union workers marched on Bank of America offices, the Harvard Club and the Statehouse to protest the nation's burgeoning student debt crisis. They said total outstanding student loans exceed credit card debt, increase by $1 million every six minutes and will reach $1 trillion this year, potentially undermining the economy.

"There are so many students that are trying to get jobs and go on with their lives," said Sarvenaz Asasy, of Boston, who joined the march after recently graduating with a master's degree and $60,000 in loan debt. "They've educated themselves and there are no jobs and we're paying tons of student loans. For what?"

And among the other protests in Oakland, parents and their kids, some in strollers, joined in by forming a "children's brigade."

"There's absolutely something wrong with the system," said Jessica Medina, a single mother who attends school part time and works at an Oakland cafe. "We need to change that."

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Associated Press writers Garance Burke and Marcus Wohlsen and Beth Duff-Brown in San Francisco, Mark Pratt in Boston, JoAnn Loviglio in Philadelphia, Jon Fahey and Verena Dobnik in New York and Christina Hoag in Los Angeles contributed to this report.

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