A boy, 12, was left with "serious injuries" after he was hit by a car, which then fled the scene.
Officers were called by the London Ambulance Service just after 4pm on Friday to a collision on Fort Avenue in Wembley.
The boy was rushed to a north London hospital where he remains in a critical condition, the Met Police said.
The car, a white Nissan SUV with a broken headlight, made off from the scene after it hit the youngster.
No arrests have been made.
In a statement, the Met Police said: "Police were called by the London Ambulance Service on Friday, 13 September at 16:07 hours to a collision in Grande Parade/Forty Avenue, Wembley.
"Officers attended and found a pedestrian, a 12-year-old boy with serious injuries.
"He was taken by the LAS to a north London hospital where he remains in a critical condition. His next of kin have been informed.
"The car, a white Nissan SUV (registration number unknown) with a broken headlight made off from the scene."
Anyone with information on this car or any witnesses should contact the police on 101 quoting CAD 5308/13 September.
