The 16-year-old boy was with friends near Tesco Express in Bromley Road when he was attacked at around 1.15pm on Wednesday, 20 December.
Officers responded along with paramedics from the London Ambulance Service. The teenager was taken to hospital with stab wounds.
His injuries were later assessed as life-changing.
Two 16-year-old boys were arrested nearby on suspicion of attempted murder. They were subsequently bailed.
Detectives have carried out a number of enquiries and are now appealing for anyone who was in the area and has information about the incident to come forward.
Detective Constable Jessica Flood, from the local policing team, said: “We believe there would have been other children in the area who witnessed this awful attack, as well as several other people who may have seen what happened. It is important they come forward and tell us what they saw.
“We have heard from one witness who was on a Route 136 bus which was passing the scene at the time. Were you also on that bus? Contact us if you saw anything suspicious.”
Anyone with information is asked to call 101 quoting CAD 3337/20Dec. Information can also be shared online.
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