A man suffered what police described as a serious hand injury after being attacked near Brighton Station.
The 31-year-old victim arrived at the station having been to a Christmas party in London.
A man came up to the victim and spoke aggressively to him so he pushed the man away and walked off towards the top of Trafalgar Street.
Sussex Police said: “He was closely followed by the suspect, who hit him causing him to fall to the ground, landing awkwardly and sustaining a serious hand injury, possibly a broken wrist.
“The suspect then ran off down Trafalgar Street and under the tunnel.”
The attack happened just after 2am on Friday 19 December.
The attacker was described as white, in his early twenties, 5ft 7in tall, slim, with short dark brown hair and a southern accent.
He was wearing either jeans or jogging bottoms, a hoody and had a large traveller style rucksack on his back, and may have been homeless.
PC Paul Baker said: “We are appealing for anyone who may have been in the area at that time and who may have seen or heard anything that may assist our investigation.”
Anyone with information is asked to phone Sussex Police on 101, email 101@sussex.pnn.police.uk or call the independent charity Crimestoppers anonymously on 0800 555111.
