The Downtown Berkeley BART station was closed during the protest, and the Alameda-Contra Costa Transit District bus No. 12 had to be rerouted, according to agency officials.
Police have reported fights, property damage and that protesters have thrown objects such as fireworks.
Police now say 18 people have been arrested and officers have seized items prohibited from the protest. Some type of gas was released during the 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 planned to gather at 10 a.m. and others at noon or later. At least two injuries have been reported so far to police, White said.
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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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.