OAKLAND, Calif. — Many in the crowd outside Oakland City Hall shouted "Burn it! Burn it!" as masked protesters readied to set fire to an American flag. That's when a woman emerged from the scrum, screaming for them to stop, that it would hurt the cause.

Moments later, the flames began, and suddenly a movement that seemingly vanished weeks ago was back in the spotlight, this time for an act of protest that has long divided Americans and now the movement itself.

The images of the flag-burning went viral in the hours after Saturday's demonstrations on Oakland's streets, with Occupy supporters denouncing the act as unpatriotic and a black mark on the movement. Others said called it justified.

The flag-burning, however, raised questions about whether the act will tarnish a movement of largely peaceful protests and begin to alienate people who agree with its message against corporate excess and economic inequality.

"I'm quite confident that the general view is that violence of this sort -- whether it's symbolic or otherwise -- is contrary to the spirit of the movement and should be renounced," Columbia University sociologist Todd Gitlin said.

Gitlin, who is writing a book about the movement, noted that flags have had a prominent place at the Occupy Wall Street encampments that sprang up last year. They are typically pinned to tents or waving from wooden flagpoles.

"I was thinking how they have come to embrace the American flag as a hallmark of this movement; it's very common to see American flags honoured and elevated at these encampments," he said.

Flag-burning has been a powerful symbol since the days of the anti-Vietnam War movement. Congress at the time passed a law to protect the flag in 1968, and most states followed suit.

In 1989, the U.S. Supreme Court decided such laws were unconstitutional restrictions on free speech. The court's decision set off a move in Congress to pass a constitutional amendment to prohibit desecration of the flag. An attempt in 2006 failed by only one vote in the Senate.

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

At Occupy Oakland, flag-burning is nothing new. A well-known San Francisco Bay Area activist burned three during protests that temporarily shut down the Port of Oakland in November.

Troy Johnson, an Occupy Oakland member, said he arrived just in time Saturday to watch his friend, whom he would not name in order to protect his identity, emerge from City Hall with an American flag in tow.

"He asked the crowd, 'What do you want us to do with the flag?"' Johnson recalled. "They said, 'Burn it! Burn it! Burn it!"'

As many egged on the bandanna-masked men, lighters were passed around. A photographer on assignment for The Associated Press said a woman rose from among the crowd to urge against the flag-burning. She then threw the flag to the ground and tried to put out the fire, shouting at them that it would only hurt their cause.

The fire-starter is not an anarchist, but a typical member of Occupy Oakland who feels the system has failed them, said Johnson, who pulled out his mobile phone to show his recording of the flag-burning.

"I would describe him as someone who loves his country, but also disappointed in the system that's running this country," said Johnson, who goes by the nickname "Uncle Boom" and was a sergeant in the U.S. Army.

Saturday's protest culminated in rock- and bottle-throwing and volleys of tear gas from the police, as well as the City Hall break-in that left glass cases smashed, graffiti spray-painted on the walls and, finally, the flag-burning.

Police said more than 400 people were arrested; at least three officers and one protester were injured.

Police said Monday that they were still trying to determine how many of those arrested were from Oakland. In the past, the majority of those arrested in Occupy sweeps were not Oakland residents and this has rankled city officials. Mayor Jean Quan has called on the loosely organized movement to "stop using Oakland as its playground."

Officials said vandalism and activities related to Occupy Oakland has cost the financially-strapped city $5 million since October.

Oakland Councilwoman Libby Schaaf said she was disgusted not to see the American and California flags atop the grand staircase inside City Hall on Monday. The destruction to her workplace couldn't have come at a worse time as the city is grappling with closing a $28 million budget deficit.

Protester Julion Lewis-Tatman said he led the crowd in the plaza outside City Hall, but did not take part in the flag-burning.

"I love this country to death, but burning the flag means nothing to me," he said. "We're burning down the old system and we're starting a new country."