Witnesses are being sought following the attack, which occurred at around 01:00 this morning (Sunday) when the victim and his girlfriend were walking along Quay Street past the entrance to Watchbell Lane.
The victim, a 38-year-old man, became involved in a verbal dispute with three men and it is then believed that he was assaulted.
The three men are described as white. One of the men was wearing black clothing. The second man was aged in his early twenties, around 5ft 8ins tall, thin, and wearing a dark round neck jumper. The third man was around 5ft 6ins tall and was wearing a flat cap.
The victim was taken to St Mary’s Hospital and is being treated for a serious head injury. He remains in hospital this morning.
Police are appealing for anyone who saw the three men in the area at the time, including a taxi driver who alerted security staff from a nearby pub of the incident, to call 101 quoting 44150113757.
Information can also be given anonymously by the Crimestoppers line on 0800 555 111.