A motorcyclist riding "recklessly" in a Bristol park grabbed a man's walking stick and hit him with it, according to police.
The incident happened in St George Park on the evening of Wednesday 26 May.
Avon and Somerset Police were called to the scene but could not find the rider.
A spokesperson for the force said the victim, who is in his 50s, was walking close to the tennis courts in the park at about 8.30pm.
He fell to the ground during the attack and his glasses were damaged as he was hit.
In a statement, police said the victim had "approached a man he witnessed riding recklessly on an off-road motorbike.
"The man took the victim’s walking stick and struck him with it, causing him to fall to the ground.
"He then struck the victim several more times. The victim’s glasses were damaged in the assault."
Officers attended and provided first aid, and helped to get him home. His injuries are believed to be minor.
The suspect is described as a mixed race man aged between 18 and 25, who was in a distinctive outfit.
He was wearing a bright yellow full face crash helmet, red goggles, black tracksuit bottoms with blue stripes down the sides and a grey hoodie with black elbow pads.
Police are now appealing for witnesses and have released a photo of someone they want to speak to in relation to the investigation, pictured above.