The fight between the two sides was triggered when Donald Trump's opponents held a counter-rally alongside a "Patriots Day" free-speech rally organised by the US President's supporters in California.
Clashes erupted on Saturday in a Berkeley, California park where supporters and opponents of US President Donald Trump were holding competing rallies. At least 20 arrests were made as police struggled to keep the two camps apart.
As fist fights broke out between the two sides and people threw bottles and cans over a barricade separating them, police resorted to using to an explosive device at one point in a bid to restore order.
Several people were observed with bloodied faces and minor injuries, but there was no official word on casualties from authorities. Media, citing police, reported that at least 11 people were injured.
Police said more arrests could follow after video shot during the melee was reviewed.
The trouble unfolded 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.
Between 500 and 1,000 people were in the park as the rallies peaked, according to an estimate by a Reuters reporter.
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.
"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," Tempesta said. "That's a message I can get behind."
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.
Thousands protested at the University of California, Berkeley after alt-right Milo Yiannopolous was invited to speak on campus. Berkeley has a long history of liberal activism and the University of California, Berkeley, was a centre of protests in the 1960s.
Thousands demand disclosure of Trump's taxes
Tens of thousands of people marched through midtown Manhattan and dozens of US cities on Saturday to demand Trump release his tax returns and to dispute his claim that the public does not care about the issue.
Organisers of the "Tax March" in more than 150 cities across the country and beyond wanted to call attention to Trump's refusal to disclose his tax history, as his White House predecessors have done for more than 40 years.
The marches coincided with the traditional April 15 deadline for US federal tax returns, though the filing date was pushed backed two days this year.
There were no reports of violence or arrests.