Tax March rallies in more than 150 cities, including Denver, drew thousands with a demand for President Donald Trump: to release his tax returns.
"Trump must immediately release his full tax returns, as all presidents and major-party presidential candidates have done for the last 40 years. Congress should enact legislation requiring this president and future presidential candidates to disclose their tax returns," according to a statement on the Tax March website.
Colorado rallies were planned in Colorado Springs, Grand Junction, Gunnison and Pueblo. Denver's march was the largest in the state.
In January, an estimated 100,000 people filled Civic Center Park for the Women's March that brought out millions for marches held across the globe.
That effort inspired Saturday's events and some of the Tax March allies - including MoveOn.org, American Federation of Teachers and National Domestic Workers Alliance - were partners during the Women's March.
More than 150 Tax March rallies were planned in cities such as Washington, New York, Boston and West Palm Beach, Fla. near Mar-a-Lago where Trump will spend his Easter weekend.
Denver's event began at 10 a.m. with speeches at the Greek Theater in Civic Center Park followed by a march north into downtown before looping back at the park.
Denver organizers said Tax March has a permit with the city and people should expect traffic delays in the area.
Speakers in Denver included Rep. Jared Polis D-Colorado, ACLU of Colorado executive director Nathan Woodliff-Stanley, state representatives Joseph Salazar, Chris Hansen and Edie Hooton.
"We need to know if the president is personally profiting from the policies he is proposing," a statement reads on the Tax March website, "And because we want know if he has contributed his fair share of taxes."
On the campaign trail, Trump insisted he would release his tax returns once an audit was complete, then dismissed the option once in office.
Sean Spicer said on Tuesday, "We filed our financial disclosure forms the other day in a way that allows everyone to understand."
Tax Day typically falls on April 15, but this year's federal tax deadline is April 18 because it fell on a weekend and Monday is a holiday in Washington.