Murder squad detectives are investigating what sparked a knife fight that left one teenager dead and three with stab wounds.
The 15-year-old schoolboy, named locally as Kasey Gordon, suffered fatal injuries during a violent scuffle in Tottenham, north London.
He was making his way home from Park View Academy when the fight erupted outside a takeaway in West Green Road at 3.45pm yesterday.
Three other teenagers, aged 14, 15 and 16, were taken to hospital by ambulance with non-life-threatening injuries.
Witnesses said the young men could be seen throwing kicks and punches at an older man armed with a knife.
Police arrested a 33-year-old man at the scene and he is being held in custody at a north London police station.
A flood of online tributes were paid to Kasey today as word spread of the schoolboy's death.
Writing on Twitter, Ama Rock said the stabbing was an "awful reality check" and the victim was a "good person".
She added: "Today I lost a precious friend. I don't know how to grieve. This is just an awful shock."
Johelle Tapper said: "RIP Kasey Gordon. You'll be so so missed. I can't believe you're gone. I'm feeling it."
At the scene of the attack today, a police cordon spanned a long stretch of West Green Road and traffic was being diverted.
A number of shops were forced to close, including a supermarket, a travel agent, a bar and a laundrette.
A bunch of yellow roses laid at the scene had a message which read: "To my bro Kasey.
"You didn't have time to grow into a man! I loved you! I still love you! God bless you. RIP. From your sister Dione."
A tribute Facebook page was set up with a photograph of the dead teenager with a shaven head and lines shaved into his eyebrow.
The image was overlaid with the message: "RIP, gone but never forgotten. You will always be in our hearts. Born a soldier, died a soldier."
The website described Kasey as an "awesome guy" and added: "Everyone is so glad they met you."
Friends said the victim's full name was Kasey Orlando Gordon.
Taxi driver Yilmaz Yuce, who works for West Green Cars, said he saw police officers storm into a Turkish cafe on the road.
He said about 50 minutes later a man in his 30s was led away by police "with blood on his face and hands".
He claimed the man, a regular visitor to the Narin Buffet cafe, was "mentally ill" but people thought he was "harmless".
He said: "At about 4pm six or seven police officers rushed into the cafe. Teachers and some students went with them but waited outside.
"They were in there for a long time - about 50 minutes - I got called off on a job but I heard that when they took him away he had blood on his face and hands.
"I heard that some boys, maybe not even these boys, started a fight with him and then he decided to hang around with a knife."
Eyewitness Jean-Pierre Laville, 30, said: "I was in the internet cafe and saw a fight going on between three boys and a man on the pavement across the road.
"The man looked like he was Greek, he was short and stocky.
"There were punches being thrown and I saw him stabbing - it was very quick.
"Two of them collapsed on the ground, and he just walked off up the road. He had blood on his face.
"There were loads of people around, they were just frozen at what had happened."
Barman Tom Bridger, 36, said: "I came out of my flat at four o'clock to go to the shops and saw a boy lying on a stretcher.
"Ambulance people and police officers were pumping his chest trying to save him. I've heard there was a fight."
A Met spokesman said the victim died shortly after 4.30pm and the other three were taken to hospital.
The 14-year-old suffered a puncture wound, the 16-year-old suffered superficial injuries and the 15-year-old had a slash wound to his face.
A post-mortem examination will take place at Haringey Mortuary. The inquiry is being led by Detective Chief Inspector Tim Duffield.
Anyone with information can contact the incident room on 020 8358 0400, or Crimestoppers anonymously on 0800 555 111.
Dozens of tearful children laid flowers beside a tree in the school grounds.
A group of girls hugged each other. A card left with the flowers read: "Kasey I never knew it was possible to cry so much. This has come as such a shock."
Another said: "The way they have taken you from us isn't fair.
"I will never hear your voice again, see you smile or hear you sing."
It is understood that pupils at the school have been given the option of going home if they wish.