A police vehicle was involved in a collision with another car in Draycott Close, near to the junction with Edmund Street shortly before 1.30pm on Tuesday.
The driver of a police vehicle was rushed to hospital and remained there suffering potentially life-changing injuries.
Another officer was also in hospital with injuries that are not believed to be life-threatening or life-changing.
Two other officers have since been discharged with minor injuries.
The driver of the other vehicle, a man aged in his 20s, was taken to hospital with injuries not believed to be life-threatening.
He was arrested on suspicion of attempted murder.
Another man who was also in the vehicle is believed to have fled the scene, and inquiries to locate him were ongoing, police said.
The Met's Department of Professional Standards has been informed.
Anyone who witnessed the incident is asked to call 101, quoting reference CAD 3656/20 Oct, or contact Crimestoppers on 0800 555 111.
