Officers were called around 10pm on Saturday (October 31) to a report of a 16-year-old boy being assaulted.
It happened between 9.30pm and 9.50pm in Selden Park, Littlehampton Road. The boy was punched and kicked by a group of nine people who had been letting off fireworks in the park.
A member of the public intervened and the group ran off. The victim suffered cuts and bruises to his face, hands and knees during the assault.
Anyone who saw what happened, or who may have any other information about the assault, is asked to contact police online or by calling 101 quoting serial 1652 of 31/10.
Alternatively, you can contact the independent charity Crimestoppers anonymously on 0800 555 111.