A young man suffered a fractured skull in the early hours of Saturday morning when he was beaten up by a gang after a night out in Londonderry.
Detectives are investigating the serious assault in the Waterloo Place area of the city.
The young man, who is in his early 20s, was initially attacked by one person.
It is believe the assailant may then have been joined by others.
The PSNI said the victim is in a serious condition in hospital.
Witnesses in the Waterloo Place and the Waterloo Street area between 1am and 1.30am have been urged to come forward.
© Press Association