Former US President Donald Trump says he expects to be arrested on Tuesday in a case that involves him allegedly paying hush money to a porn star before the 2016 presidential election.

In a post on his Truth Social platform on Saturday, Trump called on his supporters to protest ahead of his alleged arrest.

"Leading Republican candidate & former President of the United States of America will be arrested on Tuesday of next week. Protest, take our nation back!” Trump wrote, referring to himself and citing a leak from the Manhattan district attorney’s office.

He did not say what the charges would be.

A spokesperson for the district attorney’s office declined to comment.

The investigation centres on a $130,000 payment that Michael Cohen, Trump’s former personal lawyer and fixer, made to porn star Stormy Daniels in the waning days of Trump’s 2016 campaign.

Daniels, whose real name is Stephanie Clifford, said she was paid to not go public about an affair she says she had with Trump years earlier. Trump has denied the affair happened.

"So what the grand jury is investigating is whether these payments to Michael Cohen ... were actually an infringement of campaign spending funds,” said Al Jazeera’s Mike Hanna, reporting from Washington, DC.

Trump’s lawyer told the CNBC channel on Friday evening that his client would surrender to face criminal charges if he were indicted by a Manhattan grand jury.

If the Manhattan district attorney were to indict Trump, the 76-year-old would become the first former president to be charged with a crime.

Trump, a Republican, was president from 2017 to 2021 and has already declared another bid for the White House. He lost to Joe Biden in the 2020 election.

Cohen pleaded guilty in 2018 to federal campaign finance violations tied to his arranging payments to Daniels and another woman, among other crimes. He has said Trump directed him to make the payments.

Cohen testified before the grand jury on Monday and again on Wednesday, according to his lawyer, Lanny Davis. Grand jury proceedings are not public.

Daniels’s lawyer said she spoke with prosecutors last week.

The investigation is one of several legal woes Trump faces as he seeks the Republican Party’s nomination for the presidency.

"Trump is facing a series of far more consequential charges,” Hanna reported. "Yet he has chosen this case to highlight and has made a call for protests, which is very similar to the call he made back on January 6 when the riot broke out at Capitol Hill in DC and which Trump is also facing investigation for.”

The riot at the US Capitol on January 6, 2021 involved his supporters breaking through doors and windows of that building and leaving police officers beaten and bloodied as they tried to stop the certification of Biden’s election win.

Trump is also confronting a state-level criminal investigation in Georgia over efforts to overturn the 2020 results in that state.

A special counsel is currently investigating Trump’s handling of classified government documents after leaving office as well as his efforts to overturn the results of the 2020 election.

The Trump Organization, the former president’s real estate company, was convicted of tax fraud in a civil case in December, but he was not charged, prompting two prosecutors who worked on the investigation to resign.

Trump leads his early rivals for his party’s presidential nomination with the support of 43 percent of Republicans in a February Reuters/Ipsos poll, compared with 31 percent for his nearest rival, Florida Governor Ron DeSantis, who has not yet announced his candidacy.