WASHINGTON (BLOOMBERG) - President Donald Trump said he would see demonstrators gathered in Washington for a second so-called "Million MAGA March" in support of ongoing attempts to overturn his loss in the Nov 3 election.

"Wow! Thousands of people forming in Washington (DC) for Stop the Steal. Didn't know about this, but I'll be seeing them! #MAGA," the president tweeted.

Fox News showed Marine One, the presidential helicopter, flying over the march en route to Joint Base Andrews, from where Trump flew to West Point, New York, to attend the annual Army-Navy football game.

A White House spokesman earlier dismissed a claim made on social media by one of Trump's supporters, from the right-wing Proud Boys group, that he'd been invited to the White House.

"He was on a public White House Christmas tour. He did not have a meeting with the president nor did the White House invite him," said spokesman Judd Deere.

Saturday's gathering is the second in the nation's capital since Democrat Joe Biden defeated Trump and was declared president-elect more than a month ago. Organisers say on their website they expect more than 22,000 people to attend.

Among those scheduled to appear were General Michael Flynn, Trump's first national security adviser, and several conservative lawmakers.

At the first march, on Nov 14, Trump waved and smiled at supporters from his armoured limousine on the way to his Virginia golf course.

A number of downtown streets were closed by police, and people carrying pro-Trump flags and signs were seen walking adjacent to the White House complex on Saturday morning.

The march comes a day after the Supreme Court rejected a case brought by Texas seeking to nullify Biden's victory in four key battleground states, dealing a mortal blow to the president's long-shot attempt to secure a second-term.

Trump spent the morning raging against the court on Twitter, and he also vented at Attorney-General William Barr, saying he should have made a criminal investigation of Biden's son Hunter public before the election.

Several of his tweets were flagged by the social media site for containing disputed claims of election fraud.