A man is fighting for his life with third degree burns over 90 per cent of his body after catching fire in a pornography shop.

Police say the man had been watching adult videos in one of the Golden Gate Adult Superstore private booths in San Francisco when the fire ignited.

He is said to have run screaming from the booth to outside the store engulfed in flames before he collapsed.

A San Francisco Fire Department spokesman said police officers across the street from the shop were alerted to the incident when they saw a man run out of it about 6pm on Wednesday.

They managed to extinguish the flames before transporting him to St Francis Memorial Hospital's Bothin Burn Center.

He is said to be fighting for his life after suffering burns ranging from first to third-degree to more than 90 percent of his body.