(AFP) - Donald Trump said Thursday he has been indicted over his handling of classified documents after leaving office, the US ex-president's most serious legal threat yet as he pursues a second White House term.

"The corrupt Biden Administration has informed my attorneys that I have been Indicted, seemingly over the Boxes Hoax," Trump wrote on his Truth Social platform as he broke the bombshell news of a historic moment for the United States: the first time a sitting or former commander-in-chief has faced federal charges.

There was no immediate confirmation from the Justice Department.

Trump attorney Jim Trusty told CNN that his client has 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.

In his post, Trump, who is running for president again, said he has been summoned to a federal courthouse in Miami next Tuesday -- the day before his 77th birthday.

His announcement came a day after US media said federal prosecutors had informed his lawyers that he is the target of the probe into his handling of classified documents.

Trump was already the first former or sitting president to be charged with a crime -- in his case over election-eve hush money payments to a porn star who said she had an affair with him.

That indictment was handed down by Manhattan's district attorney in March.

In a statement after his initial online posts, the Trump campaign lashed out at what it called an "unprecedented abuse of power," and called for the indictment to be thrown out.

- 'Dark day' -

In a defiant video released after he shared the news, Trump repeatedly declared his innocence and framed the indictment as a form of election interference by a Justice Department "weaponized" by the Biden administration.

"They come after me because now we're leading in the polls again by a lot against Biden," Trump said in the clip.

"Our country is going to hell and they come after Donald Trump... We can't let this continue."

Fellow Republicans swiftly stood by him, including Speaker of the House of Representatives Kevin McCarthy, who has had a rollercoaster relationship with 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.

"I, and every American who believes in the rule of law, stand with President Trump."

Florida Governor Ron DeSantis, a rival for the GOP presidential nomination, echoed Trump's claims of a "weaponized" DOJ, adding on Twitter that he would "excise political bias" if elected president.

The Twitter account of Republicans on the House Judiciary Committee posted a repeated Trump slogan: "WITCH HUNT."

Special counsel Jack Smith, named by US Attorney General Merrick Garland, has been looking into a cache of classified documents that Trump had stored at his Mar-a-Lago residence in Florida after leaving the White House.

The FBI carted away some 11,000 papers after serving a search warrant on Mar-a-Lago in August, and obstruction-of-justice charges could be a result of his resisting efforts to recover the trove.

Trump eventually turned over 15 boxes containing almost 200 classified documents to the National Archives in January 2022 but was subpoenaed for any outstanding records in his possession.

When asked about the charges Thursday night, Smith's spokesman Peter Carr told AFP: "We are declining to comment."

The White House said it learned of the indictment from media coverage and a spokesperson declined to comment on the news, noting that the DOJ "conducts its criminal investigations independently," CNN reported.

- Mounting legal woes -

Some Democratic lawmakers spoke out following Thursday's revelation.

Trump's indictment "is another affirmation of the rule of law," House Democrat Adam Schiff said.

"For four years, he acted like he was above the law," he added. "But he should be treated like any other lawbreaker. And today, he has been."

Trump has repeatedly denied wrongdoing in the documents case, telling a Fox News town hall event on June 1 that "everything I did was right."

But he has openly acknowledged taking and storing the documents, undermining his lawyers' suggestion that he took the stash inadvertently in the confusion of a chaotic departure.

"This evidence just adds to the mound of stuff that already exists, and no one piece is the 'be all and end all,' but when you put them all together, the case is so strong," former Watergate prosecutor Jill Wine-Banks told MSNBC.

"You cannot imagine his getting away with this."

The latest indictment comes with Trump facing numerous other probes as he bids to be the Republican nominee to challenge Joe Biden for the presidency in 2024.

Smith is also looking at whether Trump should face charges over the 2021 US Capitol riot, and Georgia prosecutors are investigating whether Trump illegally attempted to overturn the 2020 presidential election outcome there.

Trump has already been charged with dozens of financial crimes as part of the alleged hush money scheme to silence the porn star, and is due to go on trial next March, in the middle of primary election season.

© Agence France-Presse