Police described the incident outside Cerebus Security at around 11.15 pm as a ‘serious assault’.
The 27-year-old Polish victim was rushed by ambulance to the General Hospital bleeding from his nose and mouth and with head injuries.
Several people living in the area heard the commotion of the attack and came out to see what was happening.
Five of them called 999 and told detectives that the victim was repeatedly punched in the head by a man crouching over him.
The attacker is described as being aged in his 20s, about 5 ft 8 in tall and with short blond hair.
He was wearing a white or cream-coloured top and blue jeans.