An eyewitness described how three boys fought with a man before he allegedly launched a stabbing attack in which one was killed.
A 33-year-old man was under arrest after a boy of 15 was stabbed to death in West Green Road, Tottenham, north London.
Scotland Yard said four teenage boys received stab wounds and one was pronounced dead at the scene.
A force spokeswoman said officers were called out at 3.45pm on Thursday following reports of an assault. An ambulance and the air ambulance also attended.
The young victim was pronounced dead shortly after 4.30pm and the other three were taken to hospital.
One, a boy of 14, was believed to have suffered a puncture wound but his condition was not critical.
Another, aged 16 years, suffered injuries but his condition was not critical either.
The third, aged 15, suffered a facial injury and was treated and discharged, police said.
The 33-year-old was arrested near the scene and is being held in custody at a north London police station.
The police spokeswoman said: “A boy aged 15 years was pronounced dead at the scene. It is believed he was stabbed. Detectives are retaining an open mind at this early stage re the circumstances of the incident and any motive.”