A man suffered a smashed jaw after being attacked on the seafront in Brighton on a night out to celebrate his 30th birthday.
The victim, from Hove, needed reconstructive surgery and has had plates fitted to his jaw.
His injuries were so severe that specialists will need to carry out further surgery.
Sussex Police issued an appeal for information about the attack and urged witnesses to come forward.
The force said: “The victim, a 30-year-old Hove man, went out in Brighton on the evening of Saturday (14 July) to celebrate his birthday.
“At about 1.30am on the Sunday, 15 July, he was with a group of friends on the Upper Promenade above the Shooshh club, when they were approached by a group of men, one of whom punched him in the face, breaking his jaw in several places.
“He had to have reconstructive surgery and plates fitted to his jaw, and needs further surgery in the future.
“During the incident his 20-year-old sister was pushed to the floor and suffered concussion as a result.”
Detective Constable Caroline Gridley said: “The area was busy as it was a warm night and we are appealing for witnesses to this nasty and completely unprovoked assault.
“If you saw what happened or have any other information, please contact us online or by calling 101, quoting serial 692 of 17/07.”