20 arrested as violent clashes break out between pro- and anti-Trump protesters in Berkeley, Calif.

Berkeley police say 20 people were arrested after violence broke out Saturday between groups of supporters and detractors of U.S. President Donald Trump holding rallies in downtown Berkeley, Calif.

About 200 people were at Berkeley's Martin Luther King Jr. Civic Centre 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 they were holding a "Patriot Day" at the park at noon. Counter-demonstrators then said they would hold a rally at the same place at 10 a.m.

Police put in a makeshift barrier of plastic orange poles and orange fence mesh to separate the two sides, but that quickly came down as demonstrators started punching and kicking each other, while pepper spray and firecrackers were thrown into the crowd.

The San Francisco Chronicle reports that after the barrier was put back in place, demonstrators shouted at each other from a distance.

"You go back to the '60s," shouted a pro-Trump supporter.

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"You go back to the 1400s," someone on the opposing side shouted back.

The groups then left the park and walked on Berkeley streets with police closely following them.

Authorities had said ahead of the rallies that baseball bats, sticks, flagpoles and any other item that could be used as a weapon were banned at the park. Officers on Saturday confiscated sticks, knives, flagpoles and helmets and sticks with signs on them.

The rally followed a March 4 confrontation planned by some of the same groups that left several people injured and led to arrests.

In February, protesters threw rocks, broke windows and set fires outside the UC Berkeley's student union building, where then-Breitbart News editor and provocateur Milo Yiannopoulos was set to speak. His presentation was cancelled.

Tax Day protests 

Also on Saturday, protesters took to the streets in dozens of cities across the U.S. to call on Trump to release his tax returns, saying Americans deserve to know about his business ties and potential conflicts of interest.

Organizers said the protests — dubbed the Tax March — were scheduled in nearly 150 cities, and had been planned since the women's march that took place the day after Trump's inauguration.

The scuffles in Berkeley were the only reports of violence on Saturday. 

• Trump's 2005 tax returns partially revealed after leak
• Trump tax records suggest no federal taxes for years
• How Trump plans to avoid conflict of interest 

The Washington, D.C., tax march began with a rally at the U.S. Capitol, where Senator Ron Wyden called on Trump to "knock off the secrecy." The Oregon Democrat says the people have "a basic right to know whether the president pays his fair share."

"Transparency is going to be demanded by the American people, starting with Trump's tax returns so we can understand if he's working for himself or the American people," protester Ted Livermore told the CBC News. "Congress needs to act and pass a law requiring him to do so."

For four decades, presidents and major party nominees have released some of their tax returns, with the exception of Gerald Ford. Trump's break with precedent has raised questions about possible conflicts of interest.