One of the United States' busiest sea ports reopened, a day after being closed down by anti-Wall Street protests that again turned violent in Oakland, Californian.

The Port of Oakland resumed full operations after the docks, the fourth biggest in the United States by cargo volume, were closed overnight as clashes flared, leaving more than 100 people arrested and eight hurt.

"Maritime activities at the Port of Oakland are back to normal with all terminals open for business," port authorities said in an update Thursday evening, roughly 24 hours after announcing it was shut down Wednesday.

No injuries or damage were reported there, although some protesters did briefly get into the port area. "There was a limited incursion into a private rail facility, and trespassers were escorted off peacefully," the port said.

The occupation of the port marked a significant escalation for the weeks-old Occupy Wall Street movement, which has seen thousands camp out in city squares across the country to protest corporate greed and Washington gridlock.

The loosely-organized group, which claims to represent the "99 percent" of Americans victimized by the "one percent" of business and political elites, could emerge as a significant factor in next year's presidential elections.

The Oakland port which does 59 percent of its trade with Asia sent staff home early on Wednesday as hundreds of protesters besieged the docks on the San Francisco Bay.

Police said nearly 7,000 people rallied in downtown Oakland during the day, notably targeting shuttered banks, before heading for the docks in the evening and forcing the port to close.

The protests were mainly peaceful until around midnight, when dozens of protesters in the city center hurled rocks and bottles, briefly occupied a vacant building and torched a barricade.

Riot police responded with tear gas, echoing scenes last week when dozens of people were arrested and an Iraq war veteran was critically injured as police broke up a two-week old protest camp outside City Hall.

In all 101 people were arrested during the night's clashes, Oakland police chief Howard Jordan told city council members. He was frequently drowned out by boos and hisses as he recounted police actions.

At one stage, about 200 people were found to be occupying a vacant building, police said, adding that they were armed with rocks, bottles and incendiary devices, and that officers feared they would set the building on fire.

The violent protesters appeared to be a breakaway group from the much larger Occupy Wall Street movement camped out near Oakland's City Hall, many of whom had rushed to the scene to urge calm.

The Oakland Police Department said events forced it to intervene.

"Following repeated orders for the crowd to disperse with continued assaults by rocks, lit flares, roman candles and bottles, the order was given to deploy tear gas and bean bags at approximately 12:10 am."

A spokesman for protest organizers slammed the violent minority. "They're not part of the 99 percent," Allan Brill told AFP. "They're part of the .001 percent."

"Many of us were trying to prevent window breakage and spray painting and that kind of thing," he said.

Wednesday's protests were in support of a city-wide strike call, which was followed notably by hundreds of teachers who joined demonstrators.

On Thursday afternoon, about 100 protesters milled around the relatively quiet sprawling tent encampment outside city hall.

Wooden planks covered a broken window in the Tully's coffee shop adjacent to the plaza. "We're Sorry," said one sign. "This does not represent us."

Back at the port -- which handles some $39 billion in imports and exports per year and generates tens of thousands of jobs in the area authorities underlined its importance to the unemployment-wrecked local economy.

"The need for safety and security continues today, so that people can get back to work," said the port statement.

"Continued disruptions will begin to lead to re-routing of cargo and permanent loss of jobs, a situation that would only exacerbate the ongoing economic challenges of our region."