A homeless man was knocked unconscious near the entrance of Brighton Station by someone who had been on a night out in the town.
British Transport Police officers are urging witnesses to come forward and help them bring the attacker to court.
The 47-year-old victim suffered serious facial injuries and needed hospital treatment.
Detective Constable Vanessa Cottington, the investigating officer, said: “The victim became involved in a verbal altercation with a man who was part of a group of around 20 men and women who we know had been out celebrating a 19th birthday.
“The man then suddenly punched the victim in the face in an attack that left him with serious facial injuries.
“He also lost consciousness as a result of the injuries he received to the back of his head.
“He was taken to hospital, where he was treated with stitches for a very deep cut to his mouth and chin and also for the injuries to the back of his head.”
Officers have arrested three men, who are in their late teens and early twenties and from the Crawley and Surrey areas, on suspicion of malicious wounding.
The incident took place at about 4am on Saturday 27 August.
The men have since been released on police bail until Saturday 22 October.
DC Cottington added: “This was an attack on a vulnerable man and I am urging anyone with information to contact us immediately – your help could be vital to our investigation.
“This also must have been terrifying for anyone who witnessed it, many of whom were simply trying to make their way home in the early hours after an evening out.
“Violence like this will not be tolerated and officers are viewing CCTV in the area as well as speaking to witnesses to gather a full picture of what took place.
“I’d urge anyone with information that could help our inquiries to come forward.”
Anyone with information can call the British Transport Police on 0800 405040 or the independent charity Crimestoppers anonymously on 0800 555111.