Shakilus Townsend became the latest victim of London's knife crime epidemic, the 33rd teenager murdered in Britain this year and the 18th in London alone.
Witnesses said he was attacked by a gang of around six men wearing hooded tops and balaclavas, who were allegedly "called in"by a girl the victim had argued with during the afternoon.
A senior detective described it as "another senseless incident in which a young life has been taken away by a knife".
Neighbours in Thornton Heath, south London, have told how they desperately tried to save the boy.
Dee Bamina, 35, tried to stem the bleeding from his single chest wound with a bath towel after another neighbour brought Shakilus into the communal doorway of her block of flats.
She said the boy was saying "I don't want to die" and "where's my mum? I want my mum".
Shakilus, who was known to police for his links to a street gang, died in St George's Hospital, Tooting, south London, just after midnight.
A 17-year-old boy who lives near where Shakilus was attacked said stabbings were depressingly commonplace - and often happened for no reason at all.
He said: "If he hadn't died, no one would have cared about this, it would have just been another stabbing. You grow up around here, you always see the yellow boards around and then you wake up and see 20 police vans outside.
"How are the police or the Government going to be able to sort this out if we as kids don't know why this sort of stuff's going on?"
Det Chf Insp Cliff Lyons, who is leading the investigation, said: "Given that this happened outside a block of flats in broad daylight, I am certain there are people who will have witnessed this murder.
"I would urge those people to come forward and speak to the police to help us bring justice to Shakilus's family and friends.
"To speak to an officer from the investigation team, call the incident room in Sutton on 020 8721 4205; alternatively if you wish to remain anonymous please call Crimestoppers on 0800 555 111."
Shakilus is the 18th victim this year of London's violent crime crisis. The attack comes four days after 16-year-old Ben Kinsella was stabbed to death in Islington and two French students, Laurent Bonomo and Gabriel Ferez, 23, were found stabbed and burned in New Cross.
Richard Higgins, 17, said: "The first thing I saw was loads of blood. I heard him crying for his mum over and over again.
"Even when the ambulance came he was trying to take off the oxygen mask and speak to the ambulance crew to tell them to call his mum.
"He looked terrified."
Residents said a group of about six black males and a mixed race female, dressed in hooded tops with scarves and balaclavas chased Shakilus around Beluah Crescent on bicycles.
A witnesses said the gang shouted "Get him, he's over there" before stabbing him four times in the chest.
Peter Gibbs, 60, said: "I have asked for more patrols and they never listen. There was as shooting around here last time and nobody came.
"It's a disgrace. I've seen gangs around and I am afraid. I want the police to do something about it but nobody helps. I feel terrified and so do the other residents. It's just not good enough."
Mrs Bamina, a radiographer, who lives in the block where Shakilus lives said: “I heard someone screaming and ran downstairs to find a boy lying on the ground.
“As I reached him I saw a gang of about six boys and one girl running away laughing.
“All the boys were wearing hoods and had scarves or bandanas over their faces so all you could see was their eyes."
One boy was carrying a baseball bat, she said.
“I knelt down next to the boy who was lying on the floor - he was crying and there was blood pouring from his chest.
“He kept saying 'I don't want to die' and 'where's my mum? All I want is my mum'.
“He looked so young and innocent, it was awful to see him lying there in that state.
“I asked him who his mum was but he was in too much pain to tell me.”
Mrs Bamina said she tried to stop the bleeding by pressing a towel on the wounds.
“It was clear that they were very deep wounds and I was terrified that he was going to die.
“I asked him if he knew who the boys who had attacked him were, but said he had no idea.
“He was gasping and said 'I can't breathe' and those were his last words.”
Mr Higgins, who also lives in the block of flats, said he believed that the incident started with an argument between the girl and Shakilus, who comes from Deptford, London.
The A-level student said “I heard that it all started because the lad was talking to the girl and there was some sort of dispute.
“The next thing she had called a load of her friends down to attack him.
“I was just coming into my flat when I heard someone screaming.
“I saw the boy lying on the floor and assumed he must have slipped over but when I looked closer I could see that he had been stabbed several times and was bleeding really badly.”
He added that another neighbour retrieved a 12 inch kitchen knife from the communal garden in the cul-de-sac which was handed to police.
He described the victim as an 'innocent looking' black boy with a shaven head and wearing trainers, a navy blue sweat shirt and jeans.
