A woman has been taken to hospital with life-threatening injuries after being hit by a car in Ealing.
The collision, which occurred on the junction between Moat Farm Road and Mandeville Road, has resulted in the woman being taken to a central London hospital with life-threatening head injuries.
Her condition is described as critical.
Officers added that one man has been arrested on suspicion of driving under the influence of drugs.
London Ambulance Service (LAS) said they were called at 1.24pm to the collision and sent "multiple resources" including a car, ambulance crew and incident response officer.
An LAS spokesperson added that the woman was treated for a head injury at the scene and was taken, as priority, to a major trauma centre at a west London hospital.
Transport for London (TFL) warned that several bus routes were now subject to diversions and delays.
Appeal for witnesses
Roads and Transport Police are appealing for witnesses after the driver of a silver VW Golf, a 26-year-old male, stopped at the scene.
He was initially arrested on suspicion of driving whilst under the influence of drugs and taken into custody at a west London police station.
He was further arrested on suspicion of possession of a Class A drug, believed to be heroin, and causing serious injury by dangerous driving. He remains in custody.
Detectives from the Serious Collision Investigation Unit are carrying out enquiries into the circumstances of the incident.
Any witnesses or anyone with any information are asked to call the collision investigators witness line on 020 8911 9555.