The victim was found in Albany Road near Burgess Hill at around 6.40pm.
Officers erected a cordon and at least two ambulances attended.
Reports on social media sais an air ambulance had landed nearby.
A spokesperson for the Metropolitan Police said: “Police were called at approximately 18:40hrs on Tuesday, 26 May to reports of a male found suffering a stab wound in Albany Road, SE5.
“Officers, London Ambulance Service and London’s Air Ambulance all attended the scene. The male – believed aged in his mid-teens – was taken to hospital for treatment; condition awaits.
“No arrests; enquiries into the circumstances are ongoing.
“A crime scene remains in place at the location.
“Anyone with information that could assist police is asked to call 101 or tweet @MetCC and quote CAD 7135/26May. Alternatively, contact the independent charity Crimestoppers on 0800 555 111.”
Passersby took to Twitter to share pictures of officers guarding the cordon.
One photo appears to show a person being wheeled into the back of an ambulance on a stretcher.
One man said: “Does anyone know why the Air Ambulance is trying to land on Burgess Park?”
Another said: “Loads of cops around. Sectioned off part of Burgess. Might be a stabbing.”
This is a breaking news story… more to follow.
Published at Tue, 26 May 2020 18:39:00 +0000