(AFP) - Former president Donald Trump pleaded not guilty on Thursday to criminal charges that he conspired to overturn the results of the 2020 election and defraud the American people.

Trump, the frontrunner for the 2024 Republican presidential nomination, entered his plea during a nearly 30-minute hearing at the same Washington courthouse where hundreds of his supporters have been convicted for their roles in the January 6, 2021 attack on the US Capitol.

"Not guilty," Trump said after magistrate judge Moxila Upadhyaya read the four criminal counts -- and the potential maximum prison sentences -- in the 45-page indictment brought by special counsel Jack Smith.

The 77-year-old has already been charged in two other criminal cases, and the new conspiracy charges raise the prospect of his being further embroiled in legal proceedings at the height of next year's election campaign.

Speaking to reporters at Reagan National airport before leaving Washington on his private plane, Trump said the cases brought against him were "persecution of a political opponent."

"This is a very sad day for America," he said. "This is the persecution of the person that's leading by very, very substantial numbers in the Republican primary and leading (President Joe) Biden by a lot.

"So if you can't beat him, you persecute him or you prosecute him," he said. "We can't let this happen in America."

Later on Thursday, Trump quipped on his Truth Social account that considering he had to journey "to a filthy, dirty, falling apart & very unsafe Washington" for his arraignment, "it was a very good day!"

The judge set the next hearing in the high-stakes case for August 28 before US District Court Judge Tanya Chutkan, who will preside over the eventual trial.

"I can guarantee to everybody there will be a fair process and a fair trial," Upadhyaya said.

- 'Fueled by lies' -

Security was tight around the E. Barrett Prettyman federal courthouse where the hearing was held, with metal barricades blocking access and police patrolling the perimeter.

Small groups of demonstrators holding placards milled about outside along with curious tourists.

"Jail Trump Forever," read one sign. "Trump 24," read another.

"We wanted to see it," said Dave Werner, 52, of Houston, Texas, who was visiting the capital with his son Liam, 12. "It's a little bit being part of history."

The accusations that Trump and six unnamed co-conspirators plotted to upend the 2020 election are the most serious of the cases threatening to derail his White House comeback bid.

Biden, for his part, was asked during a bike ride while vacationing in Delaware if he would follow the arraignment. His response was a curt "No."

Smith, a former war crimes prosecutor at the Hague, has charged Trump with conspiracy to defraud the United States, conspiracy and obstruction of an official proceeding and attempting to disenfranchise voters with his false claims that he won the election.

"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 linked Trump's actions following his loss directly to the attack on the Capitol, which he called an "unprecedented assault on the seat of American democracy."

"It was fueled by lies," Smith said.

- Multiple cases -

The election plot allegedly included attempts to pressure Mike Pence into throwing out Electoral College votes at the January 6 joint session of Congress called to certify Biden's win, which the vice president eventually refused to do.

Trump is scheduled to go on trial in Florida in May of next year on charges that he took top secret government documents to his Mar-A-Lago estate in Florida and refused to return them.

The twice-impeached former president also faces criminal charges in New York for allegedly paying election-eve hush money to a porn star.

He has pleaded not guilty in both those cases.

Chutkan, 61, who will preside over Trump's trial, is an appointee of former Democratic president Barack Obama and she and Trump have a legal history.

Chutkan ruled against Trump in 2021 when he filed a suit asserting executive privilege to block documents from being handed over to the congressional committee investigating the attack on the Capitol.

Chutkan has also heard nearly three dozen cases involving participants in the Capitol riot and has handed out stiff sentences.

As president, Trump was impeached by the House of Representatives for seeking political dirt on Biden from Ukraine and over the events of January 6 but was acquitted by the Senate both times.

© Agence France-Presse