A man has been taken to hospital after setting himself alight outside the Supreme Court Building in Brisbane.

A 60-year-old man was found with burns to more than 75 per cent of his body.

Witnesses say they heard a bang coming from the forecourt of the court building just after 2:00pm (AEST).

"You could smell the flames. Everyone was just going crazy. They didn't know what was happening," one said.

Bystander Tony Collins says he tried to help the man.

"Another guy came from the other side there - we tried to drag him. We dragged him about two foot."

Witnesses tried to douse the flames with their jackets before security guards arrived with a fire extinguisher.

Police are still at the scene and have cordoned off the area.