Police said that protesters are now on city streets. The downtown Berkeley BART station is closed because of the demonstration, BART officials said.
The Alameda-Contra Costa Transit District bus No. 12 has been rerouted because of the protest, according to agency officials.
Police have reported fights, property damage and that protesters have thrown objects such as fireworks.
Police now say 13 people have been arrested and officers have seized items prohibited from the protest. Some type of gas was released this morning at Martin Luther King Jr. Civic Center Park where the protest started, a police spokesman said.
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Spokesman Officer Byron White said because of the release of the gas, police put on gas masks.
White wasn't sure when protesters started gathering but some were going to gather at 10 a.m. and others at noon.
Other groups may come later, he said. He did not have any information on injuries.
The items encountered at the protest that are prohibited have included flagpoles, a knife, a stun gun, helmets and signs and flags attached to poles.
Officers continue to monitor the protest and adjust their tactics, White said.
Anyone with photos or videos of the event is asked to share them with police by sending them to policecityofberkeley.info or http://bit.ly/berkvideo.
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Both sides of the demonstration posted flyers while the city published a list of banned items -- basically anything that can be used as a weapon. Police arrested 10 people at a March 4th event and are still working to identify suspects.
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Bay City News contributed to this report.