A Good Samaritan was injured as he tried to stop another man being attacked.
The incident started with an assault on a 20-year-old man who was with friends in Medwin Walk, Horsham.
A 36-year-old man who was nearby saw what happened and went to speak to the attacker, who was with a group of friends.
The group turned on him and he was knocked to the ground and kicked in the head and body, The men ran off across Albion Way and into London Road where it is believed they got into a car.
The 36-year-old victim suffered a broken cheekbone which will require surgery.
Acting Detective Sergeant Jon Bryant said: "This was a totally unprovoked attack and I appeal for anyone with information to call me on 0845 60 70 999."
The man who attacked the 20-year-old was in his mid twenties and wearing a white shirt.
The other men in the group were of a similar age. The attack happened on Friday night.
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