Police were called to Brooke Road, E5 at 5.15pm on Sunday, 16 February following a report of a stabbing.
Officers and London Ambulance Service attended and found a man believed to be in his teens suffering a stab injury.
He has been taken to hospital where he is in a life-threatening condition.
A crime scene is in place.
If any young people have information about violence or knife crime, they can visit fearless.org/ where they can pass on information anonymously. Fearless is part of the Crimestoppers charity, and is also independent of the police.