An unidentified man who appeared to be in his thirties set himself on fire on the sidewalk in front of the White House last night and was critically burned, authorities reported.

The man, who was bundled in winter clothing and had apparently been doused beforehand with gasoline "lit himself on fire," about 10 p.m., said White House spokesman Dale Petroskey.

Flames from the blaze rose six or seven feet in the air, according to one witness. Others said they found spent wooden matches and burned bits of newspaper where the man had been standing directly in front of the main entrance to the Executive Mansion. They said they smelled fumes of a flammable liquid, possibly gasoline or kerosene.

Uniformed Secret Service officers helped witnesses extinguish the blaze, and the man was taken by D.C. fire department ambulance to the Washington Hospital Center where he was listed in critical condition early this morning with burns over 80 per cent of his body.

Authorities said they had no immediate explanation for the incident. "He didn't seem to be protesting anything," Petroskey said.

Investigators said they found no placards or other paraphernalia that might indicate a protest. None of the witnesses or investigators reported hearing the man speak.

President and Mrs. Reagan were not in the White House. They had left Wednesday for a vacation at their ranch in California.

Concepcion Picciotto, who was maintaining an antinuclear vigil at the time in Lafayette Park across Pennsylvania Avenue NW from the White House, said that she saw a man and woman on the White House sidewalk, heard a commotion and then saw a flame.

Picciotto said she shouted " 'Fire! Fire!'" and ran across the Avenue toward toward the burning man.

She said the woman who had been near the man took off her coat and that she and a man who had been part of the antinuclear vigil attempted to roll the man in the coat and on the snow-covered sidewalk to smother the flames.

A spokesman for the Secret Service said that after spotting the flames two uniformed officers who had been on guard at a White House gate ran from their post and used a blanket and a fire extinguisher to help put out the flames.

Picciotto, who spends hours each day in Lafayette Park on her vigil, said she did not recognize the man. She said the man's hands, face and legs appeared charred and swollen after the fire.

Authorities said that although they knew of no witness who had actually seen the man set himself on fire, they said the evidence was strong that he had done so.

"That seems to be what happened," said Lt. Thomas Pellinger of the U.S. Park Police, which has jurisdiction over the sidewalk outside the White House.

Although authorities and witnesses reported detecting the odor of a flammable liquid, a Secret Service spokesman said no container for such a fluid was found at the scene, indicating that the man had been doused beforehand.

Petroskey said that as far as authorities could determine the man raised or held no protest sign and "just seemed to walk up and do it."