Sussex Police said officers were called by ambulance crews to Ambrose Place at 9pm on Saturday (June 20) to reports of a 46-year-old man who had been assaulted.
He suffered serious facial injuries and was taken to hospital for treatment, police said.
Police believed the incident happened following an altercation with a group of young people.
A 19-year-old man and a 17-year-old boy were then arrested on suspicion of causing grievous bodily harm with intent and were released under investigation while enquiries continued.
Police are urging anyone who witnessed what happened to report online or call 101, quoting serial 1670 of 20/06.