A 72-year-old man has been knocked to the ground by an unknown man while attempting to board a bus in Western Australia.

The elderly man was helped to his feet by the bus driver and passengers after the altercation occurred on Hutton Street in Osborne Park at about 12.50pm on Sunday, May 12.

The man was left with cuts and grazes on his arm.

Police said the unknown man boarded the bus and walked off a short time later.

He is described as between 20 to 30 years of age with a medium build and short dark brown hair.

Detectives have released CCTV of a man they believe can assist with their enquiries.