Police were called at 8.34pm on Friday, 23 April to reports of a group of people fighting in Goldsworthy Gardens, SE16.
Officers attended along with London Ambulance Service and found a 22-year-old man with stab injuries. He was taken to a south London hospital with life-threatening injuries.
A crime scene remains in place and enquiries are ongoing. At this early stage of the investigation that has been no arrests.
Anybody with information is asked to call police on 101 quoting CAD 7778/23Apr. To remain anonymous contact Crimestoppers on 0800 555 111.
A Section 60 has been authorised in the area until 07:15hrs on Saturday, 24 April.