Oakland police and California Highway Patrol officers responded to an active sideshow in East Oakland on Sunday afternoon.
The sideshow was growing in size at 100th Avenue and MacArthur Boulevard, according to the CHP, which was called in to assist and monitor.
The sideshow initially involved about 100 cars, but it dissipated and moved toward Bancroft Avenue and Ritchie Street, CHP officials said.
There were witness reports of fireworks being set off and bottles thrown at officers, but police did not confirm those reports.
A CHP helicopter was overhead and monitoring the crowd, the CHP said.
Oakland police later Sunday said several cars involved in the sideshow were towed.
No further details were available.