Police were called at 00:39hrs on Thursday, 14 July, to reports of a man seen in possession of a firearm in Byron Road, Harrow.
Officers attended and carried out a search of the area. A man was located and arrested for possession with intent to supply Class B drugs. He was taken to a north London police station. No firearm was found.
Officers remained in the area and a short while later a man fired a shot at three officers in a marked police vehicle in Gladstone Way. He fled the scene and enquires are underway to trace him. The officers were not injured.
The suspect is described as a black male wearing a grey hooded top and jeans.
Detectives from the Trident and Area Crime Command are investigating.
At this stage no arrests have been made in relation to the shot fired at the officers.
Anyone with any information is asked to contact the investigation team via 101, or call Crimestoppers, anonymously, on 0800 555 111.