Riot police fired tear gas and arrested more than 400 people in Oakland, California, as hordes of anti-Wall Street protesters tried to take over downtown buildings including City Hall, police said Sunday.

The most violent protest of the year by an Occupy Wall Street offshoot led to what Oakland police described as "an unprecedented number of arrests" Saturday night into early Sunday.

RELATED: Videos -- Police use flash bang grenades, tear gas in Oakland

Three officers and a protester were injured in the clashes, after a peaceful march of some 1,000 demonstrators degenerated into confrontation with police in which rioters "pelted officers with bottles, metal pipe, rocks, spray cans, improvised explosive devices and burning flares," police said in a statement.

Police responded with smoke, tear gas and beanbag projectiles.

They said Occupy Oakland first targeted the vacant Henry Kaiser Convention Center before proceeding to a YMCA and then storming City Hall.

"They broke in at least briefly at City Hall and vandalized some of the art and structures," Oakland Mayor Jean Quan told a late night press briefing, adding that the months of Occupy protests in the city had become "a very tired scenario."

"Occupy Oakland has got to stop using Oakland as its playground," a frustrated Quan said. "People in the community and in the Occupy movement have to stop excuses for this behavior."

Protests against inequality and corporate influence on US politics began last September in New York and quickly spread around the country. Most of the demonstrators were removed from the streets at the end of the year.

A group of protesters burned the American flag before moving to break into historic Oakland buildings, officials said.

Ousted from its protest camp in Frank Ogawa Plaza on October 25 amid clashes with police, Occupy Oakland had earlier announced a weekend "rise up festival" to be held in an unspecified empty building.

Demonstrators at first marched toward the convention center, which includes a 5,500-seat arena, theater and event spaces.

Once there, several protesters tried to tear down a fence and occupy the building.

After the marchers were dispersed, about 500 of them regrouped and continued their protest at Frank Ogawa Plaza.

"Amazing day," read one entry on Occupy Oakland's Twitter feed (@OccupyOakland) late Saturday. "We didn't get in the building, but fought like our future depended on it. I love u all!"

In a statement earlier Saturday, police said the initial arrests took place when protesters ignored a dispersal order and assaulted officers with rocks and other objects.

"If the cops are willing to defend property over people, I think that shows where the city's priorities are," said Carla Orendorff, a student at University of California Davis.

The OccupyWallStreet website pledged mass "solidarity" protests Sunday including events in New York, Boston, and Philadelphia, with initiatives underway in several other cities like Los Angeles, Chicago, Miami and Pittsburgh.

Meanwhile in Washington, protesters from Occupy DC, facing a clampdown on their encampment Monday, stripped down for a topless street party Saturday night outside a black-tie gala for Washington's biggest movers and shakers, including President Barack Obama and First Lady Michelle.

Outside, up to 200 members of Occupy DC pogo-danced to Twisted Sister's "We're Not Gonna Take It" and Public Enemy's "Fight the Power" as police officers watched from behind barricades.

Photo: Flickr user pseudoplacebo.