Police are appealing for witnesses following a report of four men fighting around 8.30pm on Monday, September 5, in gardens near to Glamis Street, Bognor Regis.
Officers and colleagues from SECAmb provided first aid treatment to a man who was taken to hospital with serious injuries.
The public can expect to see a heighted police presence in the area and the road is likely to be closed for several hours while officers conduct their enquiries.
This is believed to be an isolated incident that does not pose a risk of harm to the wider community.
Anyone with information which could help officers with their enquiries is asked to report it to police online or call 101 quoting serial number 1277 of 05/09.
Alternatively you can report this information anonymously to the independent charity Crimestoppers on 0800 555111.