They believe the alleged assault took place sometime between 9 pm and 11 pm outside Barannes Juice Bar on the corner of Library Place and Broad Street.
A boy with blonde hair, wearing light-coloured trousers or tracksuit bottoms and a long-sleeved blue jacket or cardigan is thought to have attacked another boy with dark hair.
Three young girls and another boy were stood near the fountain opposite the juice bar at the time and police believe one of the girls was verbally abusive to a man in a black 4 x 4 who had pulled over to try and help.
Police think the blonde-haired boy then smashed the window of the bar before heading off down New Street towards Burrard Street with the group standing nearby.
If you saw what happened, give police a call on 612612 or you can call Crimestoppers anonymously on 0800 555 111.