Former US president Donald Trump has been indicted for allegedly trying to overturn the 2020 presidential election.

Special counsel Jack Smith had been investigating Mr Trump, including his role in the Capitol Hill riots of January 2021.

US media outlets are reporting Mr Trump has been named in four charges on Wednesday morning (AEST).

CNN reports that the indictment accuses Mr Trump of “spreading lies” in the months after election day 2020.

“These claims were false, and the defendant knew they were false,” it says.

“But the defendant disseminated them anyway – to make his knowingly false claims appear legitimate, create an intense atmosphere of mistrust and anger, and erode public faith in the administration of the election.”

Mr Trump’s first public reaction was a brief statement, questioning the timing of Wednesday’s indictment, maintaining the former president “has always followed the law and the Constitution”.

“These un-American witch hunts will fail and President Trump will be re-elected to the White House,” the statement read.

Mr Trump has been charged with conspiracy to defraud the US, witness tampering, and conspiracy against the rights of citizens.

Offenders convicted of the latter face a fine or up to 10 years in prison, US media says.

The charges of conspiracy to defraud the US are a nod to the Trump campaign’s creation of fake electoral certificates submitted to the US Congress.

Other charges reportedly include obstruction of an official proceeding. Some of the rioters who entered the Capitol building, including members of the Oath Keepers and military and chauvinist group the Proud Boys, have also faced those charges.

Those counts carry a maximum sentence of 20 years imprisonment.

CNN says Mr Smith has included six unindicted and unnamed co-conspirators in his indictment against Mr Trump. They included four lawyers who allegedly helped him in his efforts to subvert the 2020 election results.

Also included is one unnamed Justice Department official who “attempted to use the Justice Department to open sham election crime investigations and influence state legislatures with knowingly false claims of election fraud”, CNN says.

Mr Smith also mentions an unnamed “political consultant who helped implement a plan to submit fraudulent slates of presidential electors to obstruct the certification proceeding”.

In a statement later on Wednesday, Mr Smith said the January 6 riot was an “unprecedented assault on the seat of American democracy”, and encouraged Americans to read the entire indictment.

“It was fuelled by lies,” Mr Smith said of the January 6 storming of the Capitol.

“Lies by the defendant, targeted at obstructing a bedrock function of the US government – the nation’s process of collecting, counting and certifying the results of the presidential election.”

Mr Smith said he would seek a speedy trial “so that our evidence can be tested in court and judged by a jury of citizens”. Mr Trump is expected to appear in court in Washington DC on Friday morning (AEST).

It is the third time Mr Trump has been criminally indicted.

He already faces criminal charges in two other cases as he campaigns to regain the presidency next year. Despite his legal woes, polls show he is the leading Republican candidate for the 2024 election.

The latest charges are another extraordinary turn for the former president.

Mr Trump earlier posted on social media that he was expecting to be indicted on Tuesday night (Wednesday morning AEST).

The charges stem from Mr Smith’s investigation into allegations Mr Trump sought to reverse his loss to Democratic rival Joe Biden.

Officials have testified that Mr Trump pressured them based on false claims of widespread voting fraud.

His supporters attacked the US Capitol on January 6, 2021, in a failed bid to stop Congress from certifying Mr Biden’s victory.

Mr Trump has characterised the probe, as well as the other cases against him, as part of a politically motivated witch hunt.

-with AAP