A bus driver was slightly injured when a beer can was hurled at him through the open window of his vehicle.
The half-empty can struck the man on the side of the head.
The unprovoked attack happened as the Hoppa bus - on the 710 Laycock route - pulled up at a stop near the junction of North Dean Road and North Dean Avenue, Keighley, on Saturday night.
One man standing on the opposite side of the road moved across and threw the can, while another kicked the side of the bus.
Both men were of Asian background and aged 18-20, and carried bags. The first man had a small beard covering his chin only, and wore a baseball cap and dark bomber jacket.
Sgt Allan Gee, of Keighley police, says: "This apparently motiveless attack happened shortly after 10.30pm on a busy housing estate.
"There would most likely be quite a few people around at that time, and we would like to hear from anyone who may have witnessed the incident or can shed any light on what happened."
Motorists are warned not to leave valuables on view in unattended vehicles.
The police advice follows an increase in thefts from cars.
Sgt Gee says: "In a number of cases people have left goods clearly visible inside the vehicle, which is an open invitation to the opportunist thief.
"Motorists should try to ensure they do not leave anything of value inside the vehicle. If you do have to, make sure it is locked away out of sight."
Phone Crimestoppers on 0800 555 111 if you have information on any crime.
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