They say that a group of three men met up with two other men in Royal Square, and starting walking towards Chambers on Mulcaster Street, just after 01:25.
The Police believe the two men were then attacked, with one of them falling and hitting his head on a granite cornerstone, leaving him unconscious.
The three men then ran back through the Royal Square towards Hilgrove Street.
Anyone that may have witnessed this or who has any information regarding this incident is asked to call police on 612612 or Crimestoppers on 0800 555 111.