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, which was organised by mostly Trump supporters.
Berkeley police say they've known about the rally and counter demonstration for some time. Civic Center Park, where roughly 100 to 150 people on each side were separated by a makeshift orange barrier.
Nancy Traver, of Silver Spring, Maryland, who was at the Washington march said, "Every president, Democratic and Republican for years has showed, has revealed his tax returns, so why shouldn't Trump?"
Police said seven people had to go to the hospital before the day was done, and several people were arrested for assault with a deadly weapon, with more arrests expected as investigators review videos of the event.
"There continues to be a number of large fights, some property damage and numerous projectiles being thrown, including fireworks", Berkeley police wrote in a Nixle announcement.
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"These people create violence all the time... somebody has to stand up to them", said Alegria, who was injured in the fight and treated by Trump supporters who bandaged his head, washed off the pepper spray and gave him encouragement, saying, "You've earned your stripes, bro". "I don't agree with everything Trump says, but I don't agree with violence".
Thousands of protesters gathered Saturday in cities across the United States to pressure Trump to release his tax returns, a move of transparency he has refused to make. Prohibited items such as knives, stun guns, and flagpoles were confiscated from the protesters. One person was stabbed, police said. "Our mission is to safeguard our community while ensuring the peaceful expression of free speech", White said.
Mr Trump was the first major-party nominee in more than 40 years not to release his returns and he reneged on a campaign commitment to release them.
Trump said during the campaign that he would release his returns when the audit was complete. "We march because it is in the best interest of the American people to know what financial entanglements and conflicts of interest our leaders have". She rode a bus from New Jersey to New York City with her friend Geraldine Markowitz, 83, to take part in protests.
Senate Democratic leader Chuck Schumer, of NY, has said Trump's refusal to release his returns could hinder Republicans' prospects for a rewrite of the tax code.