Two men tending to a broken-down car were each forced to have a leg amputated after being struck by a passing vehicle.
The victims, both in their 40s, were at the side of the A406 North Circular flyover above Redbridge Roundabout in east London on Tuesday evening when they were injured.
The Met said a silver BMW 330 Sport collided with a Vauxhall Vectra, which in turn careered into the men as they stood by their vehicle.
Neither is in a life-threatening condition but police said their injuries were "extremely serious" and led to both men having a leg amputated.
A Scotland Yard spokesman was unable to confirm the identity of this car.
The two occupants of the Vauxhall were not injured.
The BMW stopped at the scene, however the male driver fled on foot before police arrived.
A 25-year-old man has been arrested on suspicion of causing serious injuries by dangerous driving.
Anyone with information is asked to contact police.