A victim in his sixties was set upon from behind as he walked towards Lake Bank.
The injuries he sustained were not life-threatening but he did lose consciousness.
No arrests have been made at this stage.
Officers want to speak to anyone who was in the area during the busy period or witnessed someone acting suspiciously. If they saw someone carrying a grey Nike rucksack which was stolen during the incident and has not been located since, it is also of interest.
Sergeant Chris Eardley said: “We are continuing to investigate this incident which was an unprovoked attack in an area we know is visited by large groups of people, especially when the weather is nice.
“We are now in a position during our enquiries to ask for the help of the local community or anyone who was in the area last weekend to come forward if they saw something, or if they have located the lost bag which will be vital in finding out who did this.
“Our neighbourhood patrols remain in place but we understand incidents like this can cause concern and remain determined to identify the suspect as we do not tolerate acts of violence like this.
“If you do have anything to share, please get in touch by calling 101 quoting incident 1256 of May 26. Alternatively, details can be passed on anonymously to Crimestoppers on 0800 555 111.”
