A 37-year-old man has set himself on fire in distressing scenes outside a New York courthouse as Donald Trump’s ‘hush money’ criminal trial was taking place inside.

The man poured a canister of flammable fluid over his body in the public park outside the Manhattan courthouse around 1.30pm on Friday after first throwing pamphlets from a bag.

He then set himself alight as horrified onlookers screamed, with one heard crying ‘Don’t do it’.

The man was engulfed in a huge ball of fire, with disturbing footage circulating online showing flames completely obscuring his body.

CNN reporters said he was burning for about three minutes in the park outside the courtroom where former President Trump’s jury selection was taking place.

The man then fell to the ground and his badly blackened and peeling body was carried away in a stretcher. Authorities said he was alive but in a critical condition in the Cornell Burn Centre.

The man has been identified as Maxwell Azzarello from St Augustine, Florida, and early inquiries suggested he was a conspiracy theorist.

Police said the man’s pamphlets were “propaganda” and “conspiracy” related, including a mention of Ponzi schemes and mob links to education.

NYPD Chief of Department Jeffrey Maddrey outlined the incident to media.

“We observe a male walk into the park. He walks into the centre of the park. When he’s in the park he starts shuffling around his clothes; he’s opens (sic) up a book bag,” he said.

“From the book bag he takes numerous pieces of paper — pamphlets — out. He throws the pamphlets throughout the park.

“And then he pulls out a canister, pours some kind of liquid on himself —the liquid we believe is an accelerant — and he lights himself on fire.

“The male, he takes a couple of steps while he’s on fire then eventually falls onto a police barrier, falls down to the ground.

“He’s on fire and some of the area in the park where the accelerant spilt is also in fire.

“Civilians, court officers, members of the police department, they run into the park. They make efforts to put him out. They use their coats, they use fire extinguishers.”

The man pulled pamphlets from a bag and threw them in the park. Photo: Getty

Police said the accelerant appeared to be some kind of alcohol-based cleaning fluid.

The man had not breached any security protocols surrounding the courthouse because the park was open to the public.

However the New York Police Department said security would be reviewed in the wake of the incident.

Trump was seen leaving the courthouse shortly after that development and later returned.

Former President Donald Trump left the courthouse after the incident. Photo: Getty

The horror unfolded shortly after jury selection for Trump’s trial was finally completed after running into problems all week.

Trump’s outsized public presence created unique problems during the jury selection process, which started on Monday.

Roughly half the first 196 jurors screened in heavily Democratic Manhattan were dismissed after saying they could not assess the evidence impartially.

The Republican politician’s criticism of witnesses, prosecutors, the judge and their relatives in this case and others has also sparked concerns about harassment, prompting Merchan to impose a partial gag order.

Merchan dismissed the juror who said she felt intimidated after family, friends and colleagues had deduced she had been selected for the trial.

“I don’t believe at this point that I can be fair and unbiased, and let the outside influences not affect my decision-making in the courtroom,” the juror said.

The judge also excused another juror after prosecutors said he may not have disclosed prior brushes with the law.

The 12-person jury consists of seven men and five women, mostly employed in white-collar professions: two corporate lawyers, a software engineer, a speech therapist and an English teacher.

Most are not native New Yorkers, hailing from across the United States and countries like Ireland and Lebanon.

Trump is accused of covering up a $US130,000 ($202,365) payment his former lawyer Michael Cohen made to porn star Stormy Daniels before the 2016 election to keep quiet about a sexual encounter she says they had a decade earlier.

Trump has pleaded not guilty to 34 counts of falsifying business records brought by Manhattan District Attorney Alvin Bragg and denies any such encounter with Daniels, whose real name is Stephanie Clifford.

Trump has pleaded not guilty in three other criminal cases as well, but this is the only one certain to go to trial ahead of the November 5 election, when the Republican politician aims to again take on Democratic President Joe Biden.

A conviction would not bar him from office.

-with AAP