OAKLAND, Calif. (AP) — It started peacefully enough: A midday rally at City Hall and a march. But as the day wore on, Oakland was hit by the most turbulent protests in weeks as Occupy demonstrators clashed repeatedly with police, leaving more than 400 people arrested.

The demonstrations in downtown Oakland broke a lull that had seen just a smattering of people taking to Oakland's streets in recent weeks for occasional marches that bore little resemblance to the headline-grabbing Occupy demonstrations of last fall.

That all changed Saturday with 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.

AP photos showing the flag burning — including images of masked protesters touching off the blaze, a woman urging protesters not to burn it, and another of an officer stomping out the fire — drew attention on social networking sites.

At least three officers and one protester were injured. Police spokesman Sgt. Jeff Thomason said there were more than 400 arrests on charges ranging from failure to disperse to vandalism,

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.

The US Flag has a special meaning in my life. To me, it symbolizes everything I've spent the last 10 years wearing a uniform for, plus the 6 further years I'll be signing for in 2 weeks. It reaches me at almost a spiritual level you can say, no matter what I'm feeling when I look at it, everything just stops and a sense of strong pride fills me. It helps me realize that no matter how things are in life, what that flag represents something that I will always be proud to say I've devoted my life to protecting. It's probably the closest thing that I have to a spiritual guide in my life.

When I see actions like this, it makes me wonder how I can justify putting people like that before it. To continue protecting their rights to tread over everything that I hold dear in life. I know it is my responsibility to, but when I truly look within myself, I find that I cannot. I cannot truthfully stand up and say I defend the right of these people to deface America.

While I hold my head high with the duty I have and will continue to faithfully serve the good citizens of America, I just do not think I can blindly extend that umbrella to all American citizens anymore...