At least 21 people were arrested as Trump supporters and opponents clashed Saturday at a park in Berkeley, California, police said.
Eleven people were injured, with seven transported to the hospital in unknown condition, Berkeley Police spokesman Byron White told CNN.
"A large number of fights have occurred and numerous fireworks have been thrown in the crowds," Berkeley police said in a statement. "There have also been numerous reports of pepper spray being used in the crowd."
CNN affiliate KPIX reported that Trump supporters planned a "Patriot Day" rally at noon and counter-protesters showed up a few hours earlier.
Hundreds of people had gathered in Civic Center Park. Police set up a barrier of orange mesh fence to separate the two sides but it quickly fell down as protesters started fighting, KPIX said.
Video showed crowds spilling into nearby streets. Scuffles broke out, fireworks, bottles and traffic cones were thrown into crowds and dumpsters and trash cans were hauled into streets. One man set afire a red "USA" hat and held it overhead.
Police donned gas masks as they used pepper spray on the crowd. A Berkeley station for BART, the mass transit system, was shut down because of the disturbance, CNN affiliate KRON said.
Police said they confiscated prohibited items including hand-held flagpoles, a knife, a stun gun, helmets and signs, and flags attached to poles, KRON reported.
A police officer was treated and released from a hospital after someone threw either pepper spray or tear gas into the crowd, White said. He added that a person in the crowd was treated and released after being sprayed with "bear spray," an irritant spray used to deter aggressive bears in wilderness areas.
An even larger protest erupted in February when people took to the streets to protest an appearance by right-wing commentator Milo Yiannopoulos at UC Berkeley. The university said more than $100,000 in damage was done.
In early March, 10 people were arrested when pro- and anti-Trump groups fought in Berkeley.
Protests were held in dozens of cities across the United States on Saturday for the anti-Trump "Tax Day," but the Berkeley rally was about free speech, according to local news reports.