Former US President Donald Trump has been criminally charged by a federal investigation into his alleged efforts to overturn his 2020 election defeat.

He is accused of four counts, including conspiracy to defraud the US, tampering with a witness and conspiracy against the rights of citizens.

The indictment caps an inquiry into events surrounding the riot two-and-a-half years ago at the US Capitol.

Mr Trump, 77, who is running for president again, denies wrongdoing.

He is already charged in two other cases: with mishandling classified files and over a porn star's hush-money payment.

The investigation has focused on Mr Trump's actions in the two-month period between his election loss and the riot in Washington DC, where supporters of his stormed Congress as lawmakers certified Democratic President Joe Biden's victory.

The man leading the inquiry, US justice department-appointed special counsel Jack Smith, said on Tuesday evening: "The attack on our nation's capital on January 6, 2021, was an unprecedented assault on the seat of American democracy.

"As described in the indictment it was fuelled by lies."

Mr Smith wrapped up his brief statement by pledging to seek "a speedy trial", while emphasising that the defendant "must be assumed innocent until proven guilty".

Mr Trump is due to appear in court on Thursday in Washington DC.

The 45-page indictment lists six unnamed co-conspirators: four lawyers, a justice department official and a political consultant.

The court document accuses Mr Trump of a "conspiracy to impair, obstruct, and defeat the federal government function through dishonesty, fraud and deceit".

About Mr Trump's allegations of voter fraud, prosecutors say: "These claims were false and the defendant knew that they were false."

They also say that after failing to convince Vice-President Mike Pence to attempt to block the congressional certification of Mr Biden as president, Mr Trump continued - including on the day of the riot - to try to cling to power.

"As violence ensued, the Defendant and co-conspirators exploited the disruption by redoubling efforts to levy false claims of election fraud and convince members of Congress to further delay the certification based on those claims."

The indictment also lists the numerous US officials and senior Trump campaign workers who, it says, informed the outgoing president that he had lost and that there was no evidence of voter fraud.

Mr Trump is currently the frontrunner in the Republican party's contest to pick its next presidential candidate. The winner will challenge the Democratic nominee, probably 80-year-old President Biden, in November 2024.

The Trump campaign said in a statement that Tuesday's indictment amounted to election interference.

"The lawlessness of these persecutions of President Trump and his supporters is reminiscent of Nazi Germany in the 1930s, the former Soviet Union, and other authoritarian, dictatorial regimes," said the campaign.

It added: "These un-American witch hunts will fail."

Dozens of top Trump administration officials and advisers were interviewed as part of the investigation, including former Vice-President Pence and former Trump lawyer Rudy Giuliani.

Those who have testified before the grand jury said they were asked about efforts by Mr Trump's team to organise slates of "fake electors" who would claim he had defeated Mr Biden in seven key states.

Prosecutors in the state of Georgia are also investigating the former president on similar grounds, focusing on whether he illegally pressured officials there to discard Mr Biden's poll victory.

A decision by prosecutors in Atlanta on whether to indict Mr Trump is expected this month.

In June, Mr Smith's team charged Mr Trump in a different case with mishandling classified documents. A trial on that indictment is due to begin in a federal court in May 2024 - months before the presidential election.

Mr Trump has also been charged in New York City with falsifying business records over 2016 hush money payments to adult film star Stormy Daniels.

Recent opinion polls show his support among Republican voters has risen in the past four months, despite the charges.

In addition to the federal inquiry, state prosecutors have also investigated Republicans who are accused of helping the Trump campaign's efforts to prevent Mr Biden from taking office.

On Tuesday, state prosecutors in Michigan charged a former Republican attorney general candidate and another Trump supporter with tampering with voting machines in an effort to prove that Mr Trump had lost due to widespread voter fraud.

The riot at Congress led to Mr Trump's second impeachment in the House of Representatives - making him the first US president ever to be impeached twice.