One witness cradled gravely-wounded 16-year-old Shakilus Townsend in her arms as he said: "I don't want to die."
Police said he was attacked by a masked gang armed with knives and baseball bat in Thornton Heath, south London.
Officers said they were hunting three black teenagers who ambushed Shakilus, known as Shaki to his friends, stabbing him in the chest and beating him around the head.
The youths had pulled up their hoods and covered their faces in bandanas for the planned attack.
They also want to trace a black girl, aged in her mid-teens and wearing a floral dress, who stood back with up to eight others and watched the sickening attack.
Police have recovered two knives, one of which was described by a witness as 18 inches long.
The death brings the toll of teenagers killed in knife attacks in the capital this year to 18.
The detective leading the investigation said it was "another senseless incident in which a young life has been taken away by a knife".
It came as three teenagers appeared in court in the north of the city accused of stabbing 16-year-old Ben Kinsella to death at the weekend.
Tunisian Hamouda Bessaad, 34, was stabbed to death in Old Kent Road on Monday, while Dee Willis, 28, died after a knife attack in Peckham the next day.
Meanwhile Metropolitan Police Deputy Commissioner Paul Stephenson gave details of a 75-man taskforce dedicated to targeting gang members.
He said tackling knife crime was the force's "number one priority" and officers would continue the tough stop-and-search regime to flush out those carrying weapons.
Shakilus died at St George's Hospital, Tooting, south London, just after midnight, after the attack in Beulah Crescent.
Police said Shakilus' parents, Nicola and Derek, and his four siblings, who live in Deptford, south-east London, were "devastated".
Residents spoke today of how they desperately tried to save the teenager as blood poured from his chest.
Dee Bamina, 35, attempted to stem the bleeding with a bath towel after another neighbour took him into the communal doorway of her block of flats.
She said: "I think a group of boys must have been after the boy. All I heard was them saying 'get him from the other side'.
"I tried to ask him his name and to tell him to calm down and lie down because he was trying to get up and go."
She said the boy was saying "I don't want to die" and "where's my mum, I want my mum". The 16-year-old told her he did not know his attackers.
Detective Chief Inspector Cliff Lyons said: "This is another senseless incident in which a young life has been taken away by a knife.
"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."
He added that the attackers were not believed to be from the area because they ran off in different directions looking for a way to escape.
Witness Richard Higgins, 17, said: "I thought he had tripped at first then I saw the knife. It was a big kitchen knife, maybe 18 inches. He had stab wounds on his stomach and one big long wound on his chest."
His mother Matilda Higgins, 43, said: "The blood drained from me when I heard what had happened on my doorstep. I am frightened every time my son leaves the house and I do not breathe until I hear his key in the lock."
A 17-year-old boy who lives near where Shakilus was attacked said stabbings were common.
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?"
Energy Minister Malcolm Wicks, in whose constituency Shakilus was stabbed, said: "This is yet another brutal assault, another tragic loss of a young life."
* Anyone with information should call the incident room in Sutton on 020 8721 4205 or Crimestoppers on 0800 555 111.
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