At least 20 people were arrested after violence broke out Saturday between groups of pro- and anti-Trump protesters holding rallies in downtown Berkeley, authorities said.
About 200 people were at Berkeley's Martin Luther King Jr. Civic Center Park when several fights broke out.
Dozens of police officers in riot gear standing nearby quickly arrested one man. Others were arrested after several skirmishes.
Trump supporters announced earlier in the week that they were holding a "Patriot Day" at the park at noon that would feature speeches by members of the alt-right.
Counter-demonstrators then said they would hold a rally at the same place at 10 a.m.
It did not go well.
Photographs of the scene showed at least two men with bloodied faces.
Officers confiscated sticks, knives, flagpoles and helmets and sticks with signs on them.