WASHINGTON, Aug 1 (AFP) - Donald Trump was indicted on Tuesday (Aug 1) over his efforts to overturn the results of the 2020 election - the most serious legal threat yet to the former president as he campaigns to return to the White House.

Trump is charged with three counts of conspiracy and one count of obstruction in the 45-page indictment brought by special counsel Jack Smith.

Trump is accused of conspiracy to defraud the United States and conspiracy to obstruct an official proceeding - the Jan 6, 2021 meeting of a joint session of Congress held to certify Democrat Joe Biden's election victory.

"The purpose of the conspiracy was to overturn the legitimate results of the 2020 presidential election by using knowingly false claims of election fraud," the indictment said.

Smith has already filed charges against Trump for mishandling top secret government documents, and the former president also faces a trial in New York for allegedly paying election-eve hush money to a porn star.

"Despite having lost, the Defendant was determined to remain in power," the indictment said.

"So for more than two months following election day on Nov 3, 2020, the Defendant spread lies that there had been outcome-determinative fraud in the election and that he had actually won.

"These claims were false, and the Defendant knew that they were false."

The indictment mentions six co-conspirators but none are identified and Trump is the only named defendant.

According to US media reports, Trump is to be arraigned in court on Aug 3.

Trump, the frontrunner for the 2024 Republican presidential nomination, lashed out at Smith, calling him "deranged" and saying he had issued "yet another Fake Indictment" to "interfere with the presidential election."

"Why didn't they do this 2.5 years ago?" Trump said in a post on his Truth Social platform. "Why did they wait so long?

"Because they wanted to put it right in the middle of my campaign," he said.

Trump has repeatedly attacked the investigation as a political "witch hunt" by the Department of Justice.