MIAMI — Donald Trump said on Thursday that he had been indicted over his handling of classified documents at his Florida estate, the former US president's most serious legal threat yet as a firestorm of criminal investigations imperil his bid for a second White House term.

It was a remarkable development that makes him the first former president in the country's history to face criminal charges by the federal government that he once oversaw.

The indictment carries unmistakably grave legal consequences, including the possibility of prison if he is convicted.

But it also has enormous political implications, potentially upending a Republican presidential primary that Trump has been dominating and testing anew the willingness of GOP voters and party leaders to stick with a now twice-indicted candidate who could still face more charges.

Within 20 minutes of his announcement, Trump began fundraising for his 2024 presidential campaign. He declared his innocence in a video and repeated his familiar refrain that the investigation is a "witch hunt". He said he planned to be in court in Miami on Tuesday — the day before his 77th birthday.

"They come after me because now we're leading in the polls again by a lot against (President Joe) Biden," Trump said in the clip. "Our country is going to hell and they come after Donald Trump ... We can't let this continue."

His allies in Congress swiftly stood by him, like House Judiciary Committee chairman Jim Jordan.

Top Republican leadership, including House of Representatives Speaker Kevin McCarthy who has had a roller coaster relationship with the former president, also rushed to defend Trump.

"Today is indeed a dark day for the United States of America. It is unconscionable for a President to indict the leading candidate opposing him," McCarthy said in a statement.

Florida Governor Ron DeSantis, a Trump opponent in the primary, condemned the indictment on Twitter, saying it represented "the weaponization of federal law enforcement".

Jim Trusty, a Trump attorney, told CNN his client had been indicted on seven charges, including the willful retention of documents in violation of the Espionage Act, making false statements, obstruction of justice and a count of conspiracy.

Though the precise details of the charges were not immediately clear, people familiar with the matter told The New York Times the conspiracy charge was related to obstruction of justice.

Prosecutors said Trump took roughly 300 classified documents to Mar-a-Lago after leaving the White House, including some 100 that were seized by the FBI last August in a search of the home that underscored the gravity of the Justice Department's investigation. Trump has repeatedly insisted that he was entitled to keep the classified documents when he left the White House, and has also claimed without evidence that he had declassified them.

XINHUA