An officer was in pursuit of a car at around 07:38hrs after it failed to stop.
The vehicle collided with street furniture in Mornington Crescent, junction with Harrington Square NW1.
A man got out of the car and was challenged by the pursuing officer, who drew his taser.
The officer detained the man who was arrested on suspicion of possession of a firearm. He has been taken to hospital prior to going into police custody.
The item has been seized for specialist examination to determine whether it is a viable firearm or a replica.
The officer, who is attached to the Parliamentary and Diplomatic Protection Command, was uninjured. There were no reports of any injuries to any members of the public.
Chief Superintendent Roy Smith said: "We are aware of a video circulating online and can assure the public we are taking care of the officer's welfare.
"His actions in the circumstances are nothing short of incredible. He had no idea if the item being pointed at him was a replica or a lethal weapon and yet he maintained his composure and detained the suspect.
"Each day police officers put on the uniform and put themselves in danger to protect Londoners. This is a perfect example of the bravery and selflessness that they exhibit so often."