Officers were called to reports of an assault in Whernside Close just before 12.15pm today (August 5).
London Ambulance Service crews also attended and found a man who had been stabbed in the arm.
His injuries were assessed as potentially life-changing.
No arrests have been made at this stage.
A spokesperson for Metropolitan Police said: “Police were called at 12.14pm s on Tuesday, 5 August following reports of an assault on Whernside Close, SE28.
“Officers attended with the London Ambulance Service and a man in his 20s was taken to hospital with a stab wound to his arm.
“His injuries are potentially life-changing.
“Officers remain in the area and no arrests have been made at this stage.
“A scene is in place at the location.
“Anyone with information is asked to call 101 or ‘X’ @MetCC and quote CAD3172/5AUG.
“You can also provide information anonymously to the independent charity Crimestoppers on 0800 555 111.”
