AP Photo/The Tribune, Bay Area News Group, Jane Tyska

Oakland police arrested about 300 Occupy Oakland protesters Saturday, after demonstrators broke into a vacant convention center, a YMCA and City Hall, where they burned an American flag.

Police fired tear gas, flash grenades and bean bags in an attempt to control the crowd after some protesters threw rocks, flares, bottles, and other objects at police officers.

Police arrested about 20 people for throwing objects.

"I did see some rocks - I saw some projectiles, I didn't see who got hit or how many people," said Oakland police officer Anthony Rachael.

Later in the day, police rounded up several others for failing to disperse.

Kristin Hanes of ABC News San Francisco station KGO was one of several reporters briefly arrested.

"We were all walking down the same street and they kind of - they came at us from both sides, issued us a dispersal notice but there wasn't really, you know, anywhere to go," said Hanes.

"They kind of herded us all into this little area, pushed up to this building and I heard a bullhorn saying that you are being arrested and we are essentially surrounded, kind of pushed together like sardines here," Hanes added.

Hanes was eventually released and de-handcuffed.

Police say at least three officers were hurt at the protests.

Oakland mayor Jean Quan said people have to stop making excuses for violent behavior.

"Occupy Oakland has got to stop using Oakland as its playground," said Quan at a press conference Saturday night.

"Once again, a violent splinter group of the Occupy movement is engaging in violent actions against Oakland," Quan said.

Quan is not the only city leader who is fed up.

"It's an escalation that in my opinion, basically amounts to domestic terrorism where people start taking buildings and costing the city an incredible amount of resources," Oakland City Councilman Ignacio De La Fuente said.

Saturday was not the first time Occupy Oakland protests have turned violent.

In November, clashes broke out between demonstrators and police trying to evict them from a city plaza.During the protests, 24-year-old two-tour Iraq veteran Scott Olsen's skull was fractured by a rubber bullet or tear gas canister.