A man was shot dead when armed officers swooped on a suspected plot to spring a prisoner from court.
He was killed just yards from Wood Green Crown Court in north London after Metropolitan Police officers surrounded a car.
Scotland Yard said four men between the ages of 19 and 31 were arrested on suspicion of conspiracy to facilitate the escape of an individual from lawful custody.
A 30-year-old and a 19-year-old were held in the Bracknell Close area, while a 31-year-old and a 25-year-old were detained in nearby Barratt Avenue. They are being held at separate police stations.
The man who was killed is believed to have been near the back entrance to the court building when he was fired upon.
Scotland Yard said the operation was not related to terrorism, but would not confirm whether the man was shot by police.
Armed police wearing balaclavas and in unmarked 4x4 vehicles carried out the "intelligence-led" operation at around 9am today.
Witness Nick Lindsay, a workman who was surfacing a road nearby, said the man who was shot was sitting in his car when police moved in.
He told Sky News: "The police all came down, they all jumped out of cars with their guns and said 'Get down', and then there was a smash, then the gunshot went off. By that time police officers grabbed us to move us out of the way."
Mr Lindsay said the incident was over in 10 to 15 seconds, but the man had been in the car for around 30 minutes before the police swooped.
He said: "He was parked there for easily half an hour, I would have said, because he was there when we first turned up this morning. He must have been there at least half an hour, minimum."
An air ambulance was called to the scene, where police earlier cordoned off a section of road around the junction of Bracknell Close and Olympus Grove, near Winkfield Road.
Paramedics fought to save the man but he died before he could be taken to hospital.
A white forensics tent was erected near an ambulance in the residential street, which is lined on either side with cars, and plain clothes officers were seen with evidence bags.
The Directorate of Professional Standards - the Metropolitan Police's internal watchdog - is investigating, along with the Independent Police Complaints Commission (IPCC).
IPCC representatives will attend a post-incident debrief, in which the officers involved will give an account of what happened during the shooting.
It comes just days after Scotland Yard launched a major crackdown on gun and knife crime in the capital.
The incident is the fourth time in recent months that the IPCC has investigated police-related shootings in London.
Joseph Hive, 28, was tasered a number of times and shot by police in Brent on October 8. He was later charged with robbery and other offences.
James Fox, 43, was shot dead by police at a block of flats in Enfield, north London, on August 31. Armed officers stormed the sixth floor after receiving reports that a man with a gun was threatening to shoot someone, and a non-police firearm was later recovered.
And the IPCC is also investigating an incident on August 21 when a 34-year-old man was shot and injured by officers in south Clapham after a stand-off lasting several hours. A firearm was later recovered.