In a statement, police said the man was sprayed with a fire extinguisher. “There was no ignition,” it added.
Police said there were no injuries at the scene, adding that the London Ambulance Service was on hand to assess the man. He was detained under the Mental Health Act and taken to a central London hospital.
According to the statement, the London Fire Brigade made the area safe by dispersing the suspected flammable liquid. The scene was reopened to the public around midday local time.
Huw Merriman, a Conservative member of parliament, said he saw the man pouring “what smells like petrol on himself.”
The area around Parliament has been the scene of protests both opposing and supporting Brexit since the UK’s EU referendum in June 2016.