OAKLAND, California (AP) ― For weeks the protests had waned, with only a smattering of people taking to Oakland’s streets for occasional weekend marches that bore little resemblance to the headline-grabbing Occupy demonstrations of last fall.

Then came Saturday, which started peacefully enough ― a midday rally at City Hall and a march. But hours later, the scene near downtown Oakland had dramatically deteriorated: clashes punctuated by rock and bottle throwing by protesters and volleys of tear gas from police, and a City Hall break-in that left glass cases smashed, graffiti spray-painted on walls and an American flag burned.

More than 400 people were arrested on charges ranging from failure to disperse to vandalism, police spokesman Sgt. Jeff Thomason said. At least three officers and one protester were injured.

On Sunday, Oakland officials vowed to be ready if Occupy protesters try to mount another large-scale demonstration. Protesters, meanwhile, decried Saturday’s police tactics as illegal and threatened to sue.

Mayor Jean Quan personally inspected damage caused by dozens of people who broke into City Hall. She said she wants a court order to keep Occupy protesters who have been arrested several times out of Oakland, which has been hit repeatedly by demonstrations that have cost the financially troubled city about $5 million.

Quan also called on the loosely organized movement to “stop using Oakland as its playground.”

“People in the community and people in the Occupy movement have to stop making excuses for this behavior,” she said.

Saturday’s protests ― the most turbulent since Oakland police forcefully dismantled an Occupy encampment in November ― came just days after the announcement of a new round of actions. The group said it planned to use a vacant building as a social center and political hub and threatened to try to shut down the Port of Oakland for a third time, occupy the airport and take over City Hall.

After the mass arrests, the Occupy Oakland Media Committee criticized the police’s conduct, saying that most of the arrests were made illegally because police failed to allow protesters to disperse. It threatened legal action.

“Contrary to their own policy, the OPD gave no option of leaving or instruction on how to depart. These arrests are completely illegal, and this will probably result in another class action lawsuit against the OPD,” a release from the group said.

Deputy Police Chief Jeff Israel told reporters late Saturday that protesters gathered unlawfully and police gave them multiple verbal warnings to disband.

“These people gathered with the intent of unlawfully entering into a building that does not belong to them and assaulting the police,” Israel said. “It was not a peaceful group.”

Earlier this month, a court-appointed monitor submitted a report to a federal judge that included “serious concerns” about the department’s handling of the Occupy protests. Police officials say they were in “close contact” with the federal monitor during the protests.

The national Occupy Wall Street movement, which denounces corporate excess and economic inequality, began in New York City in the fall but has been largely dormant lately. Oakland, New York and Los Angeles were among the cities with the largest and most vocal Occupy protests early on. The demonstrations ebbed after those cities used force to move out hundreds of demonstrators who had set up tent cities.

In Oakland, social activism and civic unrest have long marked this rough-edged city of nearly 400,000 across the bay from San Francisco. Beset by poverty, crime and a decades-long tense relationship between the police and the community, its streets have seen clashes between officers and protesters, including anti-draft protests in the 1960s that spilled into town from neighboring Berkeley.

Before the Occupy movement spawned violence, mass arrests and two shutdowns of the Port of Oakland, the city was disrupted by a series of often-violent demonstrations over a white Bay Area Rapid Transit officer’s fatal shooting of an unarmed black man named Oscar Grant on New Year’s Day 2009.

Occupy protesters have invoked Grant’s memory, referring to the downtown plaza named after Frank Owaga, the city’s first Asian-American councilmember, by renaming the former space they occupied with tents as Oscar Grant Plaza. Hundreds of Occupiers again descended on the plaza to reflect on Saturday and discuss what’s next.

Quan, who faces two mayoral recall attempts, has been criticized for past police tear-gassing, though she said she was not aware of the plans. On Saturday, she thought the police response was measured, adding that she has lost patience with the costly and disruptive protests.