A late-night knife attack in Edgware Road left a 17-year-old boy lying in a pool of blood in the entrance of a barber's shop.
The teenager was attacked near the junction with Star Street at about 11.30pm yesterday, police said this morning.
Part of the road was shut off until about 4.30am.
One shopkeeper said he had been told the stabbing happened after a fight.
"It's madness," he said. "Now and then people fight, but I haven't seen anything to this extent."
A witness who travelled past on a bus moments later called the scene "chaos".
"The police had tape up," she told the Standard.
"There was a man lying down on his back in a shop entrance. I didn’t see his face because he was lying with his feet out and his head was inside the shop.
"He didn’t look that old. I didn’t see the wound, but there was blood everywhere. He was lying in a pool of blood – I didn’t know if he was dead.
"There was a woman screaming and crying. The store was open – his body was halfway inside the door, with just the feet poking out."
Horrified workers this morning arrived at nearby shops to discover a trail of blood 30 yards long being cleaned off the pavement by council workers.
Ahmed Abid, who owns Princes Salon – where the boy collapsed after the stabbing – said he had not witnessed the incident, but that colleagues today told him the boy "couldn't move, like he was finished".
He alleged the fight had broken out in a different shop nearby before the victim, seeing the lights on at the salon, had staggered into the doorway and begged for help.
"There's a section for a mobile shop in front of the shop," he said. "He saw people standing in the street buying sim cards and just he [lay] in front of the door. He was bleeding and he couldn’t move, like he was finished. He lost a lot of blood.
"Everybody wanted to help him but he was bleeding a lot and people were scared."
A London Ambulance spokeswoman said: "We were called at 11.31pm to reports of a stabbing on Edgware Road at the junction with Star Street.
"We sent an ambulance crew, a duty officer and the medical team from London’s Air Ambulance in a car to the scene.
"We treated one patient, reported to be a teenage boy. He was taken to hospital as a priority."
No one has yet been arrested over the stabbing.
