A solo motorcycle officer was in pursuit of a car at around 7.38am on Friday, 4 February after it failed to stop.
The vehicle collided with street furniture in Mornington Crescent, near the junction with Harrington Square NW1.
A man got out of the car and was challenged by the pursuing officer, who drew his Taser.
The man reached for his waistband and retrieved what appears to be a black firearm. He dropped it before quickly picking it back up and aiming it directly at the officer.
The officer detained the man. Taser was discharged and the man was arrested on suspicion of possession of a firearm. He has been taken to the hospital prior to going into police custody.
The item has been seized for examination – at this early stage it is thought to be a replica.
The officer was uninjured. There were no reports of any injuries to any other members of the public.
Firearms officers responded due to the nature of the incident and police remain at the scene.
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.”