Police used an explosive device in an effort to restore order after about 1,000 people from two rallies gathered in Martin Luther King Jr Civic Center Park in Berkeley on Saturday.
Fist fights broke out between the two sides and people threw bottles and cans over a barricade separating them.
Several people were seen with bloodied faces and minor injuries. Local media reported that at least 11 people were injured.
The melee was sparked when hundreds of Trump opponents staged a counter-rally alongside an event billed as a "Patriots Day" free-speech rally and picnic, organised by mostly Trump supporters.
Among the Trump opponents were some counter-protesters dressed in black and wearing masks.
The other side included self-described "patriots" and "nationalists", Trump supporters, free speech advocates, and other groups.
Daryl Tempesta, 52, who said he served in the US Air Force near the end of the Cold War, went to the rally to show his support for Trump.
He said: "As a veteran, I like the track America is on, and that Trump is willing to stand and say we are still America and we are not going to be globalist, we're not going to be a communist country.
"That's a message I can get behind."
A weekly farmers market was cancelled ahead of the rally due to concerns about violence.
Even so, a stall selling fresh vegetables was open for business amid the fist fighting, explosions from firecrackers and smoke wafting through the air.
"We decided to show up anyway because this is our livelihood," said Tim Mueller, a farmer who owns the stall, surrounded by protesters on both sides.
At least 100 people from both camps eventually moved out of the park and into one of the city's main intersections, where they continued to fist fight, hurl insults and chant at each other.
The police presence was light there, and only two or three officers were seen near the crossroads.
Berkeley has a long history of liberal activism and the University of California, Berkeley, was a center of protests in the 1960s.