Most of the people at the protest said they heard about it through social media.
Protesters in Raleigh, North Carolina, said they suspect that Trump's returns might show he has paid little or nothing to the government he now heads, or that he was indebted to Russian, Chinese or other foreign interests. "Today across America we are taking the gloves off", said Sen.
Silverman said she felt "beholden to be an active citizen". "We need a president who works for all Americans - and a tax system that does, too", said the organizers of the "Tax March" initiative on its website.
Protest organizers say Americans deserve to know about Mr. Trump's business ties and potential conflicts of interest. There were also large rallies in NY and Los Angeles.
The protests were timed to coincide with the traditional April 15 deadline for U.S. tax filings (though this year the deadline falls on April 18).
"Thanks to Trump, I think that releasing your taxes when you run for president now has to be a law", said New Yorker Marni Halasa, 51, who arrived in a tutu and leggings made of fake dollar bills and holding a sign that read 'Show Me The Money!'
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.
During his candidacy he said he wouldn't release those returns because he was being audited.
Trump refused to make his tax information public during his White House bid, breaking with decades of tradition.
Debra Messing, 48, and Sarah Silverman, 46, proved they were fired up to join the march in New York City. However, tax experts have said that an audit doesn't preclude the release of tax returns.
The idea of the Tax March protests originally came about after Jennifer Taub, a law professor at Vermont Law School, saw a video of White House advisor Kellanne Conway saying Trump would not release his tax returns because voters "didn't care." Almost everyone carried a sign, expressing opinions from "Grifter In Chief" and "Investigate Trump" to "Not A Paid Protester".
The demonstrators were marching to City Hall.
Hundreds of protesters gathered in front of New Mexico's state Capitol Saturday afternoon to demand that President Donald Trump produce his tax returns.
Rallies took place in almost 150 cities, including New York, Boston, Sacramento and San Francisco. Watch the Washington, D.C. rally here.
"Disclosing tax returns is the very lowest ethical bar for a president, and we are going to insist that he clear it", Senator Ron Wyden, an Oregon Democrat, told the crowd in Washington.