Critics have raised questions about what Trump's tax returns say about his net worth and various business ties.
The largest march took place in Washington, where protesters marched from the Capitol to the White House, but sizable anti-Trump demonstrations were also staged in Los Angeles, Chicago, Seattle and Austin, Texas.
Trump has made similar claims before: first on November 10, following his presidential win, when he said that it was "very unfair" that 'professional protesters, incited by the media, are protesting, ' and again on February 3, after passing his first executive order on immigration.
"We as Americans have the right to know".
"If I decide to run for office, I'll produce my tax returns, absolutely", Trump told Irish TV in 2014. Activists, comedians and a state senator spoke at a large march of thousands in New York City.
"I think that the American public know clearly where he stands", Spicer said.
Organizers say similar Tax Day protests are being held in about 150 cities to demand that Trump release his tax returns.
"It's possible that he can now show his taxes, because his current taxes are not being audited", said Demerlis, who carried a sign saying, "Follow the Money".
Trump's audit excuse for not releasing his taxes fails its own audit. "It's been a longstanding tradition for presidents to undertake that responsibility and Mr. Trump is not".
Republican Sen. Tom Cotton of Arkansas tried to defend Donald Trump's refusal to release his tax returns - even though an audit wouldn't prevent him from doing so - and an angry crowd of town hall attendees was having none of it. "Ron Wyden called on Trump to 'knock off the secrecy".
Democratic U.S. Sen. Elizabeth Warren, of MA, on Friday posted an online video urging Congress to force Trump to release the returns.
Thousands of people took part in the rallies that Trump called "small", including a march to his Mar-a-Lago resort in South Florida, where the president usually stays for the weekend.
The Tax Foundation adds up all federal taxes - not just income taxes, the returns for which are due today - plus state and local taxes in calculating Tax Freedom Day, divides that by the total personal income earned by Americans then factors in the days in a year.