Northolt Road was closed for more than two hours after a police car answering an emergency call was in collision with another vehicle.
The two-car smash left a post box overturned after the Vauxhall vectra police car and silver Honda crashed at 9.17pm.
An ambulance and fast response car were sent to the road accident, which took place outside Sainsbury's supermarket, in South Harrow.
The driver and passenger of the police car were taken to Northwick Park Hospital, in Watford Road, where they were treated for minor injuries and discharged.
Daniel Smith was walking the area and saw the aftermath of the crash.
The 61-year-old said: "I walked past at 8.30pm and it was fine, but I came back an hour later and the street was cordoned off.
"There were buses in a queue up the road and the police car was completely smashed at the front.
"It was lucky nobody was standing there or they would have been killed."
Northolt Road was closed between Scarsdale Road and Corbins Lane following the crash and later reopened.
An investigation has been launched by the Collision Investigation Unit in Alperton.