The incident happened in Tindal Street near Langton Road at around 1pm today (August 10).
Officers and London Ambulance Service crews attended and found a boy who had been stabbed.
He was taken to hospital where his condition was assessed as potentially life-threatening and life changing.
Two boys, aged 14 and 15, have been arrested on suspicion of grievous bodily harm.
They remain in police custody.
A crime scene remains in place and enquiries are ongoing.
Detective Superintendent Lindsey Billany, of the Met’s local investigation team in Lambeth, said: “My thoughts are with the victim, who is currently receiving treatment in hospital and we are supporting their family.
“This is understandably a concerning incident for the community. We quickly arrested two suspects and our officers are working at pace to continue to investigate.
“We urge anyone who has any information or witnessed the incident to report it to us.”
Anyone with any information is asked to call 101 or post on X @MetCC quoting CAD 3350/10Aug.
