US President Donald Trump lashed out at the protesters who took part in marches across the country yesterday to demand that he release his tax returns, declaring on Twitter that "the election is over!"

Trump's comments followed a nationwide Tax March that drew thousands of people to dozens of cities on the country's traditionally recognised deadline to file taxes, April 15.

As a candidate, Trump declined to voluntarily release his tax returns - a practice followed by other presidential hopefuls since the 1970s - claiming he couldn't do so because he was under audit. Democratic nominee Hillary Clinton hammered him on the subject.

"I did what was an almost an impossible thing to do for a Republican - easily won the Electoral College! Now Tax Returns are brought up again?" Trump said in one tweet.

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In another, he suggested that someone "should look into who paid for the small organised rallies yesterday," adding: "The election is over!"

The main march unfolded in Washington, where protesters gathered in front of the Capitol and then marched west along Pennsylvania Avenue.

In South Florida, activists marched to Trump's Mar-a-Lago resort , where the President is staying this weekend. Thousands more gathered at a large march in New York, where activists, comedians and a state senator spoke. Many of the protests featured an inflatable chicken, a mascot of sorts for the march, in a bid to mock Trump's unwillingness to share his returns.

Besides responding to the Tax March, he fired off a tweet about China and North Korea, wished his supporters a happy Easter and said the United States had no choice but to build a stronger military "than ever before".