A man was stamped on the head and robbed during a "sudden and violent attack" on a bus in Wembley, police say.
Scotland Yard have released a CCTV image of a man they want to trace in connection with the incident.
The attack took place on a route 483 bus in Ealing Road in Wembley, at around 8.15pm on July 19.
The victim, a 33-year-old man, had just boarding the bus when a man attacked him and stole his wallet.
He fought back and during the pair ended up at the back of the bus where the suspect forced him down into a seat.
The suspect then knocked the victim to the floor and stamped on his head before fleeing the scene.
The victim was left "severely shaken" after sustaining cuts and bruising to his head but did not require hospital treatment.
PC Nick Kortright, from the Roads and Transport Policing Command, said: "Although the injuries sustained were not serious ones, this was a sudden and violent attack that left the victim severely shaken.
"We need the public's help to identify the man in the image - please contact us as soon as possible if you have any information."
Anyone who has information should call 101 or tweet @MetCC quoting reference CAD 7931/19July. Alternatively, you can contact the independent charity Crimestoppers anonymously on 0800 555 111.
There have been no arrests; enquiries continue.