Detectives are appealing for information and witnesses to come forward after a shop worker was attacked and left in a critical condition in Mill Hill.
Police were called by the London Ambulance Service at approximately 23:45hrs on Saturday, 6 January to reports of an injured man on The Broadway, NW7.
Officers attended and established that the man was an employee at a local shop who had become involved in an altercation with three male suspects before being assaulted.
The man, aged 49, was taken to a central London hospital where he remains in a critical condition.
His next of kin have been informed.
Another shopworker was also attacked but suffered minor injuries that did not require hospital treatment.
The three black male suspects - all believed to be youths - ran away from the scene shortly after the attack.
A crime scene remains in place.
DI Ian Lott from the Homicide and Major Crime Command said: “What we know now is that two shopworkers 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.”
Anyone with information is requested to call police on 020 8358 0200 or Crimestoppers anonymously via 0800 555 111.
You can also tweet information to police via @MetCC
There have been no arrests and enquiries continue.