A murder investigation has been launched after a shop worker was fatally attacked in a row over cigarette papers.
The victim, named in reports as father-of-two Vijay Patel, was critically injured with one blow on Saturday after refusing to sell the items to three teenage boys at a north London shop because of their age.
Scotland Yard said a 16-year-old boy had been arrested on suspicion of murder after the shop worker succumbed to his injuries in hospital on Monday afternoon.
"At this stage we believe this to be an unprovoked spontaneous incident sparked entirely by refusal to let the suspects buy what they wanted," said Ian Lott, a detective inspector from the Metropolitan Police's homicide and major crime command.
"A man has lost his life for no reason other than trying to uphold the law."
The group entered the convenience shop on The Broadway in Mill Hill late on Saturday and attempted to buy a number of items, including the cigarette papers, which they were denied.
A police spokesman said: "The three were unhappy about the decision and became aggressive, threatening to vandalise the shop.
"As such the shopkeeper and victim went outside after them to ensure no damage was done to the shop. The victim was struck once by one of the teenagers, causing him to fall to the floor."
The shop's owner suffered minor injuries when he was punched by the teenagers, who fled the scene.
Members of the public stepped in to help Mr Patel, giving him first aid before the arrival of police and paramedics, who were called at about 11.45pm.
Scotland Yard said officers found a man lying on the pavement outside the shop and he was rushed to hospital by the London Ambulance Service.
Detectives appealed for help identifying the suspects and for anyone who may have mobile phone footage or images of the incident to contact them.
They have also been conducting door-to-door enquiries and gathering CCTV from the scene.
The suspects were described as three black males, with one wearing a red sweatshirt, black jeans and white trainers, another wearing a dark-grey or black tracksuit and black trainers and the third a dark-coloured hooded sweatshirt and black jeans.
The group were seen running off along The Broadway, in the direction of Mill Hill Broadway Station, after the attack, police said.
"If you know who they are, or where they are, please get in touch and tell us their names," Mr Lott said.
"If you saw the three running away from The Broadway and where they went let us know. I would like to thank the witnesses who have already come forward and told us what they saw.
"However, I would still urge anyone who is yet to speak with us, or who has captured some of the incident on camera, to come forward to help us.
"If you saw something that may help but haven't yet spoken to police then please get in contact with us."
Anyone with any information about the identity of the three suspects or their current whereabouts should contact 020 8358 0200, or call Crimestoppers anonymously on 0800 555 111.