OAKLAND, Calif. - For weeks the protests had waned, with only a smattering of people taking to Oakland's streets for occasional 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 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 behaviour," she said.
Saturday's protests - the most turbulent since Oakland police forcefully dismantled an Occupy encampment in November - came just days after the group announced a new round of actions. The group said it planned to use a vacant building as a social centre 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, and they 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 latest protests.
Social activism and civic unrest have long marked Oakland, a 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 neighbouring 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 police 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.
Police maintained a guard at City Hall overnight, and dozens of officers were on the scene Sunday.
"They were never able to occupy a building outside of City Hall," Interim Police Chief Howard Jordan said Sunday. "We suspect they will try to go to the convention centre again. They will get not get in."
City officials said they will call for mutual aid from other police jurisdictions if needed.
Quan, who faces two 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.
She also said she hopes prosecutors will seek a stay-away order against protesters who have been arrested multiple times.
"It appears that most of them constantly come from outside of Oakland," Quan said. "I think a lot of the young people who come to these demonstrations think they're being revolutionary when they're really hurting the people they claim that they are representing."
Saturday's events began midday when a group assembled outside City Hall and marched through the streets, disrupting traffic as they threatened to take over a vacant convention centre.
The protesters then walked to the convention centre, where some started tearing down perimeter fencing and "destroying construction equipment" by the convention centre shortly before 3 p.m., police said. The number of demonstrators swelled as the day wore on, with afternoon estimates ranging up to 2,000 people, although city leaders say that figure was much closer to several hundred.
A majority of the arrests came after police took scores of protesters into custody as they marched through the city's downtown, with some entering a YMCA building, Thomason said.
Michael Davis, 32, who is originally from Ohio and was in the Occupy movement in Cincinnati, said Sunday that Saturday was a hectic day that originally started off calm but escalated when police began using "flash bangs, tear gas, smoke grenades and bean bags."
"What could've been handled differently is the way the Oakland police came at us," Davis said. "We were peaceful."
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.