A police spokesman said officers were called to Crowhurst Recreation Ground, Sompting, at 9.45pm where a fight had broken out between two groups of youths – one containing 15 youngsters and another containing four.
One of the youths in the smaller group, a 17-year-old boy, was hit on the head with a bottle by a boy from the larger gang, said police.
An 18-year-old girl was then assaulted by the same boy and another girl from the larger group, before having items stolen.
Police believe the violence then spread into Sompting Mini-Market in Seadown Parade.
The spokesman said a man entered the mini-market at 10.30pm and stole some alcohol.
The 47-year-old shopkeeper gave chase to the junction of Silverdale Road and Browness Avenue, where he was hit around the head with a bottle, stabbed in the back and stabbed in the stomach.
Police said a 14-year-old boy from Southwick was arrested on suspicion of grievous bodily harm with intent and robbery.
A 17-year-old girl from Worthing was also arrested on suspicion of grievious bodily harm with intent, said police.
The paid have both been bailed until December 24.
Detective Constable Katie Clack said: “The shop owner suffered a wound to his back and a head injury and thankfully his injuries aren’t as serious as first thought. However this is a violent assault on a shopkeeper and we are appealing to anyone to might know something.
“Those responsible are believed to be part of the large group from Crowhurst Recreation Ground.”
Anyone who saw what happened or has any information which could help is asked to contact police online quoting serial 1284 of 28/11.