Officers were called to Queensway, between the Sunken Gardens and Fitzleet House, shortly after 8pm on Monday, April 12 to reports of a large fight.
A teenage boy was found with a serious facial injury and was taken to hospital for treatment.
Police investigating the matter are keen to speak to anyone who saw what happened, or who may have captured the incident on their mobile phone.
Officers would particularly like to identify a bald man dressed all in dark clothing who was seen walking away from the location in the direction of Queensway.
Anyone who witnessed the assault or who has any other information is asked to contact police online or by calling 101 quoting serial 1230 of 12/04.
Alternatively, you can contact the independent charity Crimestoppers anonymously on 0800 555 111.