The incident happened around 1.30pm yesterday.
There was a disturbance involving a large group of youths, believed to be from a nearby high school, and a male cyclist.
During this altercation a 15-year-old boy, who is not believed to have been part of the group, was injured after being struck to the head with a bike lock as he was walking with friends towards the Raeburn Hotel.
He was taken to hospital for treatment and officers are now conducting enquiries to trace the cyclist who left the scene before police arrived.
The cyclist is described as a white man, 39-42 years old, with a medium build. He had a bald head with triangle-shaped sunburn or a birthmark on it.
He was wearing glasses, jeans and a pale/grey short-sleeved shirt.
Detective Constable Chris Harding said: “We’re appealing for anyone who may have witnessed this disturbance, or has information which can assist with our enquiries, to come forward.
“In particular, we are keen to trace the cyclist as soon as possible to establish all the circumstances surrounding this incident and I would urge this individual to contact police immediately.”
Anyone with information is asked to contact Police Scotland on 101 or contact Crimestoppers anonymously on 0800 555 111.