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 on Thursday.
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.
One line of inquiry is that the victim confronted the suspect over a young girl before a fight broke out and his friends joined in to defend him. It is understood the man in custody is suspected of suffering from a mental illness.
A flood of online tributes have been paid to Kasey 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 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."