A man who was threatened at gunpoint managed to escape in a passing van only to be dragged out of the vehicle and stabbed in the legs, police said.
The attack came after a blue Peugeot and a red Audi crashed on Oldham Road in the Newton Heath area of Manchester about 13:30 on Wednesday.
The victim, who was in the Peugeot, remains in a stable condition in hospital.
Both the Peugeot and Audi were abandoned at the scene.
Greater Manchester Police had been called to reports of a fight in Oldham Road but the group had left when they arrived.
The member of the public's van, which the victim had got into for his safety, was rammed by a white Volkswagen Golf as it was driving along The Gateway.
The Golf's occupants then dragged him from the van before repeatedly stabbing him and driving off.
The van driver took the injured man to Central Park Police Station on Northampton Road. From there he was taken to hospital.
Det Ch Insp Paul Walker said: "This is an absolutely terrifying sequence of events, which must have been extremely frightening for other motorists and people who were in the area at the time, especially for the member of the public who took the man to the police station."
Any reports of gun crime would be taken very seriously, he added.
He appealed for witnesses and anyone with dashcam footage to come forward.