Washington DC - Donald Trump was indicted on Tuesday 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 US and conspiracy to obstruct an official proceeding – the January 6, 2021 meeting of Congress held to certify 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.

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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 porn star Stormy Daniels.

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

"So for more than two months following election day on November 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."

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The indictment mentions six co-conspirators, but none are identified and Trump is the only named defendant. Still, observers have identified former New York mayor Rudy Giuliani as one of them.

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

Trump is scheduled to go on trial in Florida in the classified documents case in May of next year, at the height of what is expected to be a bitter and divisive presidential campaign.

Georgia prosecutors are also looking into whether Trump illegally attempted to overturn the 2020 election outcome in the southern state.

The probe was sparked by Trump's January 2, 2021 phone call with Georgia Secretary of State Brad Raffensperger, when he infamously pressured the election official to "find" 11,780 votes that would reverse his defeat to Biden in the state.