A cyclist who fell off his bike in a collision with a car punched the driver and then chased him with a knife.
Police are appealing for the witnesses to the assault outside Northwick Park Hospital, which left the victim with serious head and facial injuries.
The victim, a 36-year-old man, had dropped his wife off at the hospital in Watford Road, Harrow, on Monday, January 27 and was looking for a parking space when the cyclist hit the left-hand side of his car at high speed, knocking the wing mirror off and causing the cyclist to fall off his bike.
The cyclist asked the victim to call for an ambulance, but then began to argue with the motorist, pulling something out of his bag and punching the victim in the side of the head.
The victim ran away, but was chased by the suspect, who was armed with a knife.
The attacker then rode away on his bike.
He is described as black, aged between 28 and 35, 5ft 10ins tall, and was wearing a dark blue jacket and dark-coloured trousers.
Detective Constable Nicola Benson of Brent CID said: “This was a vicious attack involving a knife and I am appealing for any witnesses who were at the hospital on the day and witnessed the incident.
“If you know who the suspect is, call me on 020 8733 3742 or alternatively, to remain anonymous, call Crimestoppers on 0800 555 111.”