Former US President Donald Trump pleaded not guilty on Tuesday (Jun 13) to federal criminal charges that he unlawfully kept national-security documents when he left office and lied to officials who sought to recover them.

Trump's plea, entered before US Magistrate Judge Jonathan Goodman in a federal court in Miami, sets up a legal battle likely to play out over coming months as he campaigns to win back the presidency in a November 2024 election. Experts say it could be a year or more before a trial takes place.

Trump, wearing a blue suit and a red tie, frowned and leaned back in his chair but did not speak during the 47-minute hearing.

He was allowed to leave court without conditions or travel restrictions and no cash bond was required. Goodman ruled that he was not allowed to communicate with potential witnesses in the case.

Trump's aide Walt Nauta, who is also charged in the case, appeared in court alongside Trump but will not have to enter a plea until Jun 27 because he does not have a local lawyer. He, too, was released without having to post bond and was ordered not to talk to other witnesses.

Supporters chanted "We love Trump" as his motorcade departed the courthouse at 3.55pm EDT, roughly two hours after it arrived.

Trump is the first former president to be charged with federal crimes.

Authorities had prepared for possible violence, recalling the Jan 6, 2021, attack on the US Capitol, but Miami Mayor Francis Suarez told reporters that there had not been any security problems.

Trump has repeatedly proclaimed his innocence and accuses Democratic President Joe Biden's administration of targeting him.

"We've got a government that's out of control," he said. Florida's Cuban-American community is a substantial Republican voting bloc in the politically competitive state.

Special Counsel Jack Smith, who is handling the case, accuses Trump of risking national secrets by taking thousands of sensitive papers with him when he left the White House in January 2021 and storing them in a haphazard manner at his Mar-a-Lago Florida estate and his New Jersey golf club.

Those records included information about the secretive US nuclear programme and potential vulnerabilities in the event of an attack, the indictment said.

The 37-count indictment alleges Trump lied to officials who tried to get them back.

It also alleges Trump conspired with Nauta to keep classified documents and hide them from investigators. Nauta has worked for Trump at the White House and at Mar-a-Lago.

Trump's legal woes have not hurt his standing with Republican voters.

A poll released on Monday showed Trump still led rivals for the Republican nomination for the 2024 presidential election by a wide margin, and 81 per cent of Republican voters viewing the charges as politically motivated.

Most of Trump's Republican presidential rivals have lined up behind him and accused the FBI of political bias, in a sharp turn from the party's traditional support for law enforcement.

Vivek Ramaswamy, one of those candidates, said outside the Miami courthouse that he would pardon Trump if he were elected.

The case is likely to benefit Trump in the short term, with his camp spinning it as a witch hunt, said one analyst.

“There's a rally around him by those who are loyal to him and his cause,” said Quinnipiac University's political science professor, Dr Wesley Renfro.

“We’re already seeing a spike in fundraising. It's almost certainly going to help him secure the GOP nomination for the presidency.”

However, the indictment will likely haunt him later on in the election when voters go to the poll.

“Trump is not particularly popular with swing voters – the small slice of the US electorate that tends to decide who the president will be. These charges will likely make him even less popular,” Renfro told CNA938's Asia First.

In the meantime, Trump is free to campaign for the presidency and could take office even if he were to be found guilty.

Trump faces charges that include violations of the Espionage Act, which criminalizes unauthorized possession of defense information, and conspiracy to obstruct justice. He would serve a maximum of 20 years in prison if convicted.

Legal experts say the evidence amounts to a strong case, and Smith has said Trump, who will turn 77 on Wednesday, will have a "speedy" trial.

The judge assigned to the case, Aileen Cannon, was appointed by Trump in 2020 and issued a ruling in his favor during the investigation last year that was reversed on appeal. Goodman, the magistrate judge who conducted Tuesday's hearing, is not expected to play an ongoing role.

Experts say the complexities of handling classified evidence and legal maneuvering by Trump's lawyers could delay a trial by more than a year. His defense team is in flux after two lawyers quit the case on Friday.

Trump accuses Biden of orchestrating the federal case to undermine his campaign. Biden has kept his distance from the case and declines to comment on it.

In his first presidential run in 2016, Trump called for imprisoning Democratic rival Hillary Clinton for using private email while serving as secretary of state, leading to chants of "lock her up" at campaign rallies. Then-FBI Director James Comey criticized Clinton for carelessness but did not recommend criminal charges.