A man allegedly poured petrol on himself while standing next to Bexhill and Battle MP Huw Merriman outside Parliament.
Pictures shared on social media appeared to show a man being arrested next to Carriage Gates on Parliament Square.
Mr 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.”
A spokesman for the Met Police confirmed a man has been arrested.
The force said: “A man was detained in Parliament Square, Westminster, at approximately 10:45hrs today after dousing himself with what appeared to be a flammable liquid.
“No injuries. LAS (London Ambulance Service) are on scene and he is currently being assessed.”
The Met Police said the man had a lighter and was sprayed with a fire extinguisher before there was an ignition.
The man was assessed by the ambulance service and taken to a central London hospital. He was detained under the Mental Health Act.