London police said Tuesday they detained a man who was dousing himself with what appeared to be a flammable liquid outside parliament.
"The man, who had a lighter, was sprayed with a fire extinguisher," the Metropolitan Police said in a statement. "There was no ignition and the man was detained."
The man was assessed by ambulance workers and taken to hospital. His age or condition were not immediately released.
One lawmaker who was at the scene tweeted the substance "smells like petrol".
The incident came against a background of heightened tension over Britain's imminent departure from the European Union, which has divided the country.
MPs are currently debating ways to make sure Britain does not crash out of the EU without a negotiated agreement on its October 31 departure date.