Former US President Donald Trump arrived in Miami on the eve of a federal criminal indictment for keeping classified documents after his term ended, in a trial that goes far beyond the misconduct charges that pursued him in the past and which he managed to evade.

Trump will appear in court on Tuesday to answer the accusations against him that he lied and planned to keep dozens of secret documents that he transferred to his residence in Florida after leaving the White House in 2021.

The serious indictment, which Trump supporters say is politically motivated to block his return to the White House, has raised fears of unrest, as Miami police are preparing to receive protesters who may number up to 50,000 people.

But the former president, who was tried in Congress twice and who is competing for the Republican nomination for the 2024 elections, maintained a defiant tone as he prepared to become the first US president to be summoned by a federal court amid strict security measures.

"We must all be strong and defeat the Communists, the Marxists and the madmen of the Radical Left who are systematically destroying our country," Trump wrote on his social network, TruthSocial, as he headed out to spend the night at his golf club, 125 minutes from the courthouse.

Trump, the frontrunner in the 2024 Republican primaries, has confirmed that the latest indictment will not force him to withdraw from the election.

"I will never walk away," Trump told Politico on his plane to a campaign rally over the weekend.

The billionaire, who will turn seventy-seven on Wednesday, is accused of possessing government secrets, refusing to return them, and conspiring to obstruct the work of investigators who were seeking to recover them.

He is also accused of sharing top-secret information with people without security clearance, in a case more serious than any he has faced before.

The indictment includes photos showing boxes that were supposed to be in the National Archives stacked in ballrooms, bedrooms and bathrooms at Mar-a-Lago, Trump's Palm Beach residence.

The former president had dismissed the 49-page indictment issued by the Justice Department at the conclusion of a months-long investigation, describing it as "ridiculous and baseless."

Security has been stepped up around the Wilkie D. Ferguson Jr. Courthouse in Miami, with some right-wing groups planning demonstrations, for example a local branch of the far-right Proud Boys.

"We hope tomorrow will be peaceful. We encourage people to be peaceful in showing their feelings," Miami Republican Mayor Francis Suarez told reporters.

Trump is expected to travel to his golf club in Bedminster, New Jersey, afterward to reaffirm his innocence in a speech to supporters.

The former Republican president has twice been tried by Congress and accused of interfering with a federal investigation into ties between his campaign and Russia, and was convicted in a civil trial of sexual assault charges.

His company was convicted of fraud, Trump University suspended operations due to lawsuits, and his charitable foundation was dissolved by court order amid a series of controversies.

Trump's closest aides were not spared from prosecutions. His campaign chairman, vice president, personal lawyer, chief strategist, national security adviser and others were convicted of crimes related to their work with him, and some even faced imprisonment.

But Trump did not personally face criminal charges in any of these cases.

That changed dramatically in March, when the New York attorney general charged Trump with 34 counts for allegedly covering up payments to buy the silence of a porn star.

Trump's allies in Congress denounced the government's use of the judiciary as a weapon against conservatives.

Some Republicans have come under fire for rhetoric that could incite violence, such as Rep. Clay Higgins from Louisiana who told his supporters to "buckle up" and Andy Biggs from Arizona who tweeted, "Now we're at war. An eye for an eye."

The Southern District Court of Florida is known as a "rocket court," a colloquial expression used to denote courts that press for speedy justice or decide cases quickly, so that the authorities did not rule out completing the trial before the 2024 elections.

Much of the focus in the preliminary proceedings will be on Judge Eileen Cannon, a Trump appointee who was chosen for this case at random and will have a tremendous impact on how quickly things move.

Cannon's series of rulings in Trump's favor earlier in the case derailed the investigation for weeks, until a conservative appeals court ruled it had acted outside its authority.