The incident happened in Green Wrythe Lane at around 11.30am yesterday (January 30).
Officers and London Ambulance Service crews attended and found a boy who had been stabbed.
Green Wrythe Lane Carshalton stabbing: Live traffic updates
He was taken to hospital for treatment where his condition was initially unknown.
A spokesperson for Metropolitan Police confirmed today (January 31) that his condition is critical.
No arrests have been made as police enquiries continue.
A businessman, who wishes not to be named, recalled seeing an ambulance transport someone to the waiting air ambulance that had landed on Wrythe Green.
He told Your Local Guardian: “[Someone] was put in an ambulance and the ambulance drove and put him in the helicopter.
“They put him in an ambulance and drove him up to where the helicopter could land.”
The reaction from the community in response to the stabbing has been one of shock, with many expressing that they would not expect it from the area.
For Ercan at Wrythe Green Fishbar, the news filled him with dread.
He said: “Absolutely shocked. It could have been anybody. It is not something I would expect from the area.
“When I first found out I was in so much shock because my family is here as well.
“I was trying to get through to my missus, and I couldn’t get through to her and it was really bad.
“That heartbeat for five minutes when I was driving back from Sutton to here – it was terrible.
“Im shaken up. People are quite concerned now because it is not the first time.”
Officers are appealing to anyone with information to call 101 and quote CAD 2789/30Jan.
Alternatively, to provide information anonymously call Crimestoppers on 0800 555 111.
