A man was attacked by ten men armed with knives and batons as he walked alone across Forest Row cricket pitch near East Grinstead.
The incident happened at approximately 11.45pm on Thursday, April 11, police said.
The 20-year-old, who lives locally, was also robbed of his mobile phone and other personal items.
A Sussex Police spokesman said: “Police are investigating a robbery which occurred at approximately 11.45pm on Thursday, April 11, when the victim, a 20-year-old local man, was attacked as he walked alone across the village cricket pitch at Forest Row, near East Grinstead.
“A group of ten young men armed with knives and batons confronted the man, who had earlier walked through Tompsets Banks car park.
“He was beaten up and had his mobile phone and other personal items stolen.
“Immediately after the robbery the victim made his way down Highgate Road to its junction with Card Hill, followed by his attackers.
“He later attended Princess Royal Hospital, Haywards Heath, for treatment to cuts and bruises before being allowed home the next morning.”
Detective Constable Andy Mountford-Laker, the officer in charge of the investigation, said it had been an isolated incident but it was important that those responsible were identified as soon as possible.
He said: “We are keen to speak to anyone who saw what happened, who noticed the group concerned before, during or after the attack, or who may have other information, dash-cam or CCTV footage that could assist our enquiries.”
Anyone with information is asked to report details online or phone 101, quoting serial 74 of 12/04.
Alternatively, they can contact the independent charity Crimestoppers anonymously, either online or by phoning 0800 555 111.