A MAN caught fire Wednesday evening inside a San Francisco porn store and was fighting for his life in the hospital after suffering third-degree burns, KCBS-TV reported.

Arson experts said it was not clear what ignited the fire but police said the man had been watching videos in a private booth when the blaze erupted.

The man ran out the front door of the adult arcade "engulfed in flames" and was spotted by police standing across the street, a police spokesman told KCBS.

"He came out of the building already on fire," Lt. Kevin McNaughton said.

Firefighters, who luckily were only about a block away on an unrelated call, raced to the scene and doused the flames.