Officer Hugh Carew, a spokesman for the Metropolitan Police Department, said the initial report was the man was conscious and breathing. Carew could not confirm how the fire had started.
Adam Stifel saw part of the incident as he was running.
"I saw a man sitting in flames," Stifel told CNN. "He had already doused himself in gasoline, I believe."
Stifel said five or six people used their shirts to pat out the fire. He said he saw a red gasoline canister near the man.
The incident occurred near the National Air and Space Museum. The man was airlifted out to receive medical care.
The location where the man was found on fire is about six blocks from the U.S. Capitol, where a high speed car chase on Thursday ended with a woman being shot and killed by law enforcement officers.