Waitrose was cordoned off and a victim was taken to hospital with injuries after reports of a fight in Lewes.
Scene of crime officers were sent to the upmarket supermarket just after 19:00 GMT on Saturday.
Sussex Police said they were called to an altercation involving youths in the town, but did not give further details.
Officers carried out investigations inside the cordoned-off shop. Waitrose said the incident was not in the store but the victim came in seeking help.
A Waitrose spokesman said an off-duty paramedic who was in the shop at the time helped staff attend to the victim.
Staff called for an ambulance and the police and the store was closed early to allow emergency services to carry out their work.
“We are thankful that the gentleman is making a good recovery and must praise our partners, the off-duty paramedic, ambulance service and police for their swift action,” he said.
At the scene: BBC reporter Nathalie Edell
There were at least half a dozen police cars, Waitrose was shut and a wide area around it cordoned off. A police officer would not tell me what had happened but he said it was serious.
Locals arriving at the shop were being directed away, while officers were speaking to a number of young teenagers, one of whom, a girl, was crying.
Inside the cordon, a teenager was being spoken to by officers. He appeared to have been handcuffed.
At the side of the store, another teenager was being asked to empty his pockets.
A spokesman for Sussex Police said the victim’s injuries were not life-threatening.
He said officers at the scene were stood down and the cordon was later lifted at the supermarket in Eastgate Street.
He could not confirm whether any arrests had been made but said police were still developing intelligence regarding any suspects.