A 48-year-old Lancing man was repeatedly punched in the face by a teenager, suffering lip cuts and damaged teeth.
The victim was in Monks Recreation Ground, off Crabtree Lane, in Lancing at about 7.45pm on Sunday when he was assaulted.
The teenage boy attacking him was stood among a group of ten to 15 children at the time.
PC Matt Welch of Sussex Police said: "The victim suffered a nasty facial injury and we are keen to find people who saw the incident."
A 17-year-old boy from Lancing has been arrested and questioned on suspicion of assault.
He was released on bail until August 17.
Anyone with information should call 101 quoting serial 1445 of 03/08, email firstname.lastname@example.org or call the independent charity Crimestoppers anonymously on 0800 555111.