"It does appear that a man sustained self-inflicted burns," Metropolitan Police Department spokesperson Gwendolyn Crump said.
The incident happened at the Mall, the country capital's grassy esplanade, some six blocks away from the Capitol Hill, where yesterday, a woman motoroist led the authorities on almost two-mile chase, sparking a brief lockdown of the high-security complex.
The District of Columbia Fire Department spokesman Tim Wilson said the man has life-threatening injuries.
The identity of the man was not immediately known. The police is yet to reveal his name.
"I saw a man sitting in flames. He had already doused himself in gasoline, I believe," Adam Stifel, an eyewitness, was quoted as saying by the CNN.
Five or six people used their shirts to pat out the fire, he said.