A 15-year-old boy and a 21-year-old man are both fighting for life after daylight stabbings in Haringey and Croydon.
The teenage boy was found suffering stab wounds in Willan Road, on the Broadwater Farm Estate in Tottenham, on Friday morning.
The boy remains in a critical condition in hospital. His parents have been told.
Officers believe the teenager had been involved in an altercation before he was knifed. The suspect may have been riding a bicycle, the Met Police said.
Three people were arrested on suspicion of attempted murder on Friday evening.
A man in his early 20s has become the fourth person to be arrested on suspicion of attempted murder on Saturday.
Police said officers were called to the scene at 11:14am.
The boy was treated at the scene before he was taken to hospital by the London Ambulance Service.
Detective Inspector Paul Ridley of North Area CID is appealing for witnesses and members of the public "who valiantly administered crucial first aid to the victim" to come forward.
“The victim, not yet 16-years-old, sustained multiple stab wounds and is fighting for his life," he said.
“If you know anything about the circumstances surrounding the incident or if you witnessed it, please do come forward."
A 21-year-old man is also fighting for his life after being stabbed on Croydon High Street.
His phone and watch are thought to have been stolen too, police said.
Three men have also been arrested on suspicion of GBH.
A statement from MPS Croydon said: "Police were called at 17:26hrs to Croydon High Street after a 21-year-old man was found suffering from a stab injury. Officers attended with London Ambulance Service.
"He has been taken to hospital where his condition is currently being treated as life-threatening."
Anyone with information is asked to call police on 101 or anonymously call Crimestoppers on 0800 555 111.
