Firearms officers opened fire after the official car of the ambassador to Ukraine was rammed outside the embassy in west London.
An attacker repeatedly drove into the car in the incident on Saturday morning, the Ukrainian Embassy said. No injuries were reported.
The force said that, on arrival at the scene, a vehicle was driven at police officers.
Police firearms and Taser were discharged before the vehicle was stopped and a man in his 40s was arrested.
He was not injured but was taken to a central London hospital as a precaution.
The incident is not being treated as terrorist-related.
Scotland Yard said both armed and unarmed officers were deployed to the incident “as part of the protective security arrangements for London”.
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.”
A statement on the Ukrainian Embassy website said: “Around 10am on Saturday 13 April, the official vehicle of the Ambassador of Ukraine to the United Kingdom was deliberately rammed as it sat parked in front of the Embassy of Ukraine’s building.
“The police were called immediately, and the suspect’s vehicle was blocked up. Nevertheless, despite the police actions, the attacker hit the Ambassador’s car again. In response, the police were forced to open fire on the perpetrator’s vehicle.
“The culprit was apprehended and taken to a police station. No one of the Embassy staff were injured.”
A woman who works at a nearby shop said she heard shots fired twice between 10am and 11am on Saturday.
The woman, who did not wish to be named, said officers arrived “very quick”, adding: “I saw many police cars coming.”
One Twitter user claimed to hear at least 10 shots.
A silver car outside the embassy was being investigated by a forensic officer.
The vehicle, which a police source said was a Mercedes, appeared to have been backed into the black car, which was heavily scuffed on its right-hand side.