The man was taken to hospital as a precaution but was not injured, they added. It added that British police were investigating the suspect's identity and a possible motive.
Ruptly's footage from the scene shows a sedan with its doors open in the middle of the street and forensic officers examining the area.
Officers used firearms and a Taser before arresting a man on suspicion of attempted murder.
The incident is not being treated as terrorism, added police.
The vehicle has been a focus of attention for forensics officers.
Chief Superintendent Andy Walker, from the Met's Specialist Firearms Command, said: "As is standard procedure, an investigation is now ongoing into the discharge of a police firearm during this incident".
The Met said the Directorate of Professional Standards has been informed.