Donald Trump has been indicted by a Manhattan grand jury after a probe into hush money paid to porn star Stormy Daniels, becoming the first former US president to face criminal charges even as he makes another run for the White House.

The charges, arising from an investigation led by Democratic Manhattan District Attorney Alvin Bragg, could reshape the 2024 presidential race. Trump previously said he would continue campaigning for the Republican Party's nomination if charged with a crime.

In a statement, Trump said he was "completely innocent."

"This is political persecution and election interference at the highest level in history," he said, providing no evidence. Shortly after, Trump appealed to supporters to provide money for a legal defence.

The specific charges are not yet known and the indictment will likely be unsealed by a judge in the coming days. Trump will have to travel to Manhattan for fingerprinting and other processing at that point.

His lawyers Susan Necheles and Joseph Tacopina said they will "vigorously fight" the charges, while another lawyer, Alina Habba, predicted he would be vindicated.

Necheles said she did not know when Trump would surrender.

Bragg's office did not immediately respond to requests for comment. 

The Manhattan investigation is one of several legal challenges facing Trump, and the charges could hurt his presidential comeback attempt. Some 44 percent of Republicans said he should drop out of the race if he is indicted, according to a Reuters/Ipsos poll released last week.

Trump's allies and fellow Republicans blasted the indictment as politically motivated, while Democrats said he is not immune from the rule of law.

The White House declined to comment.

Outside the courthouse, four protesters silently held signs criticising Trump but there was no evidence of unrest. Authorities bolstered security around the courthouse after Trump called for nationwide protests on March 18, recalling his charged rhetoric ahead of the Jan 6, 2021, attack on the US Capitol.

Daniels, whose real name is Stephanie Clifford, has said she received money in exchange for keeping silent about a sexual encounter she had with Trump in 2006.

The former president's personal lawyer Michael Cohen has said he coordinated with Trump on the payments to Daniels and to a second woman, former Playboy model Karen McDougal, who also said she had a sexual relationship with him. Trump has denied having affairs with either woman.

"No one is above the law," Daniels's lawyer Clark Brewster said on Twitter.

Cohen pleaded guilty to a campaign-finance violation in 2018 and served more than a year in prison. Federal prosecutors said he acted at Trump's direction.

Cohen said he stood by his testimony and the evidence he provided to prosecutors. "Accountability matters," he said in a statement.

No former or sitting US president has ever faced criminal charges.

Bragg's office last year won the criminal conviction of the businessperson-turned-politician's real estate company for tax fraud.

Trump also faces two criminal investigations by a special counsel appointed by US Attorney General Merrick Garland and one by a local prosecutor in Georgia.

Trump served as president from 2017 to 2021, governing as a right-wing populist. He was impeached twice by the House of Representatives, once in 2019 over his conduct regarding Ukraine and again in 2021 over the attack on the US Capitol by his supporters. He was acquitted by the Senate both times.

Trump falsely claims that his 2020 re-election loss to Democrat Joe Biden was the result of widespread voting fraud.

He leads his early rivals for his party's nomination, holding the support of 44 percent of Republicans in a March Reuters/Ipsos poll, compared with 30 percent support for his nearest rival, Florida Governor Ron DeSantis, who has yet to announce his candidacy. Biden is expected to seek re-election.

Trump in 2018 initially disputed knowing anything about the payment to Daniels. He later acknowledged reimbursing Cohen for the payment, which he called a "simple private transaction."

Cohen testified before the Manhattan grand jury, as did David Pecker, the former publisher of the National Enquirer. The tabloid publication bought the rights to McDougal's story about her alleged relationship with Trump for US$150,000 (RM664,000) but never published it, a method known as "catch and kill" used to bury damaging information about a third party.

In the case that led to the conviction of the Trump Organization on tax fraud charges, Bragg declined to charge Trump himself with financial crimes related to his business practices, prompting two prosecutors who worked on the probe to resign.

Among Trump's ongoing legal woes are a criminal investigation led by Fani Willis, the Democratic district attorney in Georgia's Fulton County, into whether he unlawfully tried to overturn his 2020 election defeat in that state.

Special counsel Jack Smith is separately investigating Trump's handling of classified government documents after leaving office and his efforts to overturn the results of the 2020 election.

- Reuters