More than 100 protesters dressed in black have pushed their way past police barricades and into a Berkeley park.
They assaulted at least three people by punching and kicking them.
At least one of those attacked was a right-wing male protester who showed up for a "Rally Against Hate" event where anti-rally protesters vastly outnumbered the rally's supporters.
Police broke up the fights, using what appeared to be a smoke grenade to stop one scuffle.
The three people who were attacked ran behind police lines to escape.
The black clad protesters carried a large banner that identified them as anarchists.
Several people were arrested at Berkeley's Civic Center Park by police who have banned protesters from covering their faces and carrying a variety of items that could be used as weapons.
About 2,000 people gathered in what was billed as a "Rally Against Hate." Opponents of the rally far outnumbered supporters.