The boy, 16, ran for help after being knifed in the chest and back in Munster Square, Camden, last night.
Two men held him back while another thrust the foot-long blade into his body, it is understood.
During the horrific ordeal the victim, an unnamed Arsenal fan due to receive his GCSEs, banged on a door for help.
But he was cruelly turned away after the resident noticed he was bleeding, a witness claims.
And after being turned away his jacket was apparently jammed in the door.
“He banged on the door and the guy had opened the door and then closed it when they saw he’d been stabbed”
Naqeeb Khan, 44, told the Evening Standard: “He banged on the door and the guy had opened the door and then closed it when they saw he’d been stabbed.
“I called to the people to open their door to release his jacket because he was bleeding a lot. About 15 minutes later police came.
“When I was screaming, another man came out who lived a few doors away and we pulled his jacket out of the door and laid him on the ground.
“He was bleeding a lot. I didn’t speak to him a lot because I didn’t want him to waste his breath. He was dying in front of me.
“I told him to be patient. That was the best I could do.”
Another local added: “We saw three boys all wearing hoods all covered in black, the middle one had a long black machete type knife. It was really long.
“The boy that was stabbed was banging on someone’s door saying ‘help me, help me’.”
The murder is the 90th in London this year, of which 15 have been teenagers.
Met Police said they have made no arrests.
London Mayor Sadiq Khan, who has been heavily criticised for spiralling London knife crime under his reign, said: “The senseless death of a 16-year-old boy in Camden last night is another example of how violent crime destroys lives and traumatises communities.
“My thoughts are with his family and friends.”