A shop worker is on a life support machine after he was “violently attacked” when he refused to serve underage youths.
Police say two shop employees were assaulted after they “rightly refused to sell” the youths either cigarettes or alcohol.
Scotland Yard said the attack took place in the shop on the Broadway in Mill Hill, north London, at about 11.45pm on Saturday night.
A 49-year-old was taken to hospital, where he remains in a critical condition.
The other suffered minor injuries that did not require hospital treatment.
Detectives are appealing for information to trace the three black male suspects, all believed to be youths, who ran away after the attack.
DI Ian Lott said: “What we know now is that two shop workers have rightly refused to sell items to youths they believed to be underage.
“The three teenage suspects have then made threats before violently attacking both shopkeepers.
“One of the men assaulted is now on a life support machine.
“We understand that a number of people came to his aid and helped to put him in the recovery position.
“Were you there? Did you see the suspects running away?
“Some witnesses may have video footage during or after the attack and we would urge them to get in touch with investigators.”
A crime scene remains in place and the Met are urging anyone with information to call police on 020 8358 0200 or Crimestoppers, anonymously, on 0800 555 111.