Police said the 42-year-old man sustained a serious head injury after the attack in the area of Star Kebabs in Victoria Road/Station Approach, Worthing, at about 9pm.
A police spokesperson said: “The victim remains in hospital where he is being treated for his injury. At this time, we don’t believe that any weapons were used but it is unclear what actually led to the assault taking place.
“We’re anxious to hear from anyone who saw what happened or who has information about the incident and would ask them to call us as soon possible on 101, quoting serial 1481 of 3 June. Alternatively, contact the independent charity Crimestoppers anonymously on 0800 555 111.”
Police have arrested a 25-year-old man on suspicion of grievous bodily harm (GBH).