An 18-year-old suffered a broken jaw when he was attacked and robbed by a gang of youths after attending a toga fancy dress party with friends.
The victim and a 17-year-old girl, a woman, 18, and a man, 21, were assaulted after the party at Itchen Rowing Club in Southampton on Saturday.
Up to 10 youths approached them as they waited for a taxi on the corner of Canute Road at about 0100 BST.
Police said three mobile phones were also stolen.
The 18-year-old man, who was punched in the face, underwent surgery at Southampton General Hospital.
Police are appealing for witnesses.