Former US President Donald Trump said he expects to be "arrested" Tuesday over an alleged hush-money payment to a porn star in 2016 and he urged his supporters to protest, as prosecutors gave signs of moving closer to an indictment.

If indicted, he would be the first former US president to be charged with a crime, marking an explosive and unpredictable development in the 2024 White House race as Trump seeks again to clinch the Republican nomination.

"Leading Republican candidate & former President of the United States of America will be arrested on Tuesday of next week," the 76-year-old said Saturday on his Truth Social platform, adding: "Protest, take our nation back!"

Not even a conviction in the hush money case would prevent Trump from running, but an indictment could have major consequences, galvanizing his critics but also electrifying his supporters.

Republican House Speaker Kevin McCarthy reacted with fury, accusing New York prosecutors of pursuing "political vengeance" against Trump. McCarthy vowed on Twitter to launch a congressional probe of the matter.

But his Democratic predecessor, Nancy Pelosi, who was still speaker when Trump was impeached for inciting the January 6, 2021 rioting in the US Capitol, said his announcement would "foment unrest among his supporters."

"He cannot hide from his violations of the law, disrespect for our elections and incitements to violence," Pelosi added.

The current investigation, led by Manhattan district attorney Alvin Bragg, an elected Democrat, centers on $130,000 paid weeks before the 2016 polls to stop porn star Stormy Daniels from going public about an affair she says she had with Trump years earlier.

Trump has denied having had an affair with Daniels, whose real name is Stephanie Clifford, and has insisted the probe amounted to "blatant and unconstitutional" election interference and that the procedure wasn't "a prosecution, it's a persecution."

His lawyer told CNBC on Friday that Trump, if indicted, would surrender to face criminal charges.

- 'Illegal leaks' -

In his Truth Social post, written in capital letters, Trump referred to "illegal leaks from a corrupt & highly political Manhattan district attorneys office" and said the investigation was "based on an old & fully debunked (by numerous other prosecutors!) fairytale."

He doubled down in another post, assailing his successor, President Joe Biden, as "crooked" and urging his followers to "Protest, Protest, Protest!!!"

Reports said authorities were planning to increase security around the Manhattan courthouse in the coming days.

Trump's lawyer Susan Necheles indicated that his posts Saturday were based on media reports and not on any fresh action taken by prosecutors.

"Since this is a political prosecution, the District Attorney's office has engaged in a practice of leaking everything to the press, rather than communicating with President Trump's attorneys," Necheles said in a statement to AFP.

A grand jury is a citizen panel which examines evidence presented by prosecutors to decide if a charge is warranted.

The jury in New York heard on Monday from Trump's former lawyer Michael Cohen, who made the payment to Daniels. Cohen was sentenced to three years in prison in 2018 on related federal charges. He pleaded guilty but said he had been carrying out Trump's orders.

Daniels herself met with prosecutors on Wednesday and "agreed to make herself available as a witness, or for further inquiry if needed," according to her lawyer Charles Brewster.

Trump has been invited to testify, which legal experts say suggests an indictment is near.

The payment to Daniels, if not properly accounted for, could result in a misdemeanor charge for falsifying business records. That might be raised to a felony if the false accounting was intended to cover up a second crime, such as a campaign finance violation, The New York Times has reported.

- 2024 prospects -

Trump is facing several criminal probes at state and federal level over possible wrongdoing that threaten his new run at the White House.

In Georgia, a prosecutor is investigating Trump and his allies' efforts to overturn his 2020 election loss in the southern state. The grand jury in that case has recommended multiple indictments, the forewoman revealed last month.

The former president is also the subject of a federal probe into his handling of classified documents as well as his possible involvement in the January 6 rioting.

Some observers believe an indictment bodes ill for Trump's 2024 chances, while others say it could provide a huge boost.

"The arrest secures the nomination for Donald Trump," tweeted Rick Wilson, a political strategist who quit the Republican Party in protest over Trump. "The base will rally politically, and possibly physically."