Donald Trump arrived in Miami on Monday to face charges of hoarding classified documents, in a legal reckoning that goes far beyond misconduct accusations the former US president has largely ridden out until now.

Trump will appear in court on Tuesday to answer charges that he lied and schemed to hold on to dozens of government secrets he took to his beachfront mansion in Florida when he left office in 2021.

The high-stakes indictment -- which Trump's supporters say is a politically motivated bid to block their champion's path back to the White House -- has fueled fears of unrest, with Miami police bracing for crowds of up to 50,000.

The twice-impeached Republican, facing charges that can carry decades-long prison sentences, struck a defiant note as he prepared to become the first of America's 46 presidents to be arraigned in federal court.

"We must all be STRONG and DEFEAT the Communists, Marxists, and Radical Left Lunatics that are systematically destroying our Country," Trump posted on his Truth Social network as he flew to spend the night at his Florida golf course, a 25-minute drive from the courthouse.

The runaway frontrunner in the 2024 Republican primary said the latest indictment would not force him to quit the race -- teeing up a campaign like no other in history that will pit a legal contest against an electoral one.

"I'll never leave. Look, if I would have left, I would have left prior to the original race in 2016," Trump told Politico on his plane as he campaigned at the weekend.

The billionaire, who turns 77 on Wednesday, is charged with willfully possessing clearly-marked government secrets, refusing to return them and conspiring to obstruct investigators seeking to recover them.

He is also accused of sharing sensitive US secrets with people who had no security clearance, in a much more serious case than any he has previously faced.

The indictment included photographs showing boxes that were supposed to be in the National Archives stacked at Mar-a-Lago, Trump's Palm Beach residence, in a "ballroom, a bathroom and shower, an office space, his bedroom and a storage room."

The former president has rejected as "ridiculous and baseless" the 49-page indictment released by the Justice Department following months of investigation by a special prosecutor.

Security was being ramped-up around Miami's Wilkie D. Ferguson Jr. Courthouse, with several protests planned including by a local chapter of the far-right Proud Boys group.

"We hope that tomorrow will be peaceful. We encourage people to be peaceful in demonstrating how they feel," Miami's Republican mayor Francis Suarez told reporters.

Trump is expected to fly to his golf club in Bedminster, New Jersey, afterwards to restate his innocence in a speech before supporters. (AFP)