At 10:44hrs on Tuesday, 1 October Officers became aware of a man dousing himself with, what appeared to be, a flammable liquid at Old Carriage Gates, Westminster.
The man, who had a lighter, was sprayed with a fire extinguisher.
There was no ignition and the man was detained.
The London Fire Brigade (LFB) and London Ambulance Service (LAS) have attended the scene.
LFB made the scene safe by dispersing the suspected flammable liquid.
The man was assessed by LAS and taken to a central London hospital. He has been detained under the Mental Health Act.
No reports of any injuries.
The area affected has now been reopened.