Burning clothes were strewn in the park, which was locked down by authorities, while ambulances lined up nearby on standby, an AFP correspondent at the scene saw, describing a strong smell of burning chemicals.
The scene unfolded moments after the full panel of 12 jurors and six alternates was selected for the trial of the former president in a hush money cover-up case.
“A male did light himself on fire outside of the Supreme Courthouse. We’re still gathering details from the field,” said a New York Police Department spokesman.
The fire department did not respond to a request for comment, but media reported the man had been taken away for medical treatment.
The incident happened in a park opposite the 100 Centre Street courthouse, which has been used by authorities to corral protesters, both pro-Trump and anti-Trump, as well as by some members of the media.
The park, which is close to several courthouses and other buildings, is a popular local lunch spot.
Trump’s criminal trial, the first of a former president, is being conducted amid tight security in a 15th-floor courtroom swarming with Secret Service officers as well as court police.
New York’s police department had promised a major deployment to ensure the trial passes off safely, with the force’s head of intelligence John Hart calling it a “major challenge.”