A man in his early 20s has become the fourth person to be arrested on suspicion of attempted murder over the stabbing of a 15-year-old boy.
The victim is fighting for his life in hospital following the attack in Tottenham, north London, on Friday morning.
He was found with multiple stab wounds on Willan Road, in the Broadwater Farm estate, at about 11.15am.
On Friday evening two other men in their early 20s, as well as a 17-year-old boy, were also arrested on suspicion of attempted murder, Scotland Yard said.
Officers believe the boy was involved in an altercation shortly before he was stabbed, possibly by a suspect riding a bicycle.
Police would like to speak to members of the public who gave first aid to the victim at the scene.
Detective Inspector Paul Ridley, of North Area CID, said: "I am keen to hear from anyone who either witnessed, or has information concerning this attack.
"The victim, not yet 16 years old, sustained multiple stab wounds and is fighting for his life.
"If you know anything about the circumstances surrounding the incident or if you witnessed it, please do come forward.
"I would also appeal to the members of the public who valiantly administered crucial first aid to the victim to come forward to my investigation team.
"You can speak with my team or remain anonymous by speaking to Crimestoppers, either way is fine, but please do the right thing, make the call."
Anyone with information is asked to call police on 101 quoting incident number 2902/30AUG or call Crimestoppers anonymously on 0800 555 111.