The incident unfolded at around 10.45am on Tuesday and the Metropolitan Police said the man is being assessed.
London Ambulance Service also attended.
Police said the man was not injured.
Conservative MP Huw Merriman tweeted: “Man next to me at Parliament’s carriage gates appears to have poured (what smells like) petrol on himself.
“Incredibly brave response from police, who are now helping him.”
The man was spotted close to where PC Keith Palmer was murdered in a terror attack in 2017.