The incident happened in Bromley High Street at around 7.50pm yesterday (April 6).
Officers and London Ambulance Service crews attended and found a boy who had been stabbed.
He was treated at the scene before being taken to hospital where his condition was assesed as non life-threateing.
No arrests have been made as police enquiries continue.
A spokesperson for the Metropolitan Police said: “While no arrests have been made at this stage, officers understand the impact of knife crime and are working quickly to locate those responsible – including gathering witness statements and relevant CCTV footage.
“Anyone with information is asked to contact police via 101, quoting 6172/6APR.
“Those who wish to remain anonymous can go through Crimestoppers on 0800 555 111.”
A spokesperson for London Ambulance Service said: “We were called at 7.50pm on Monday 6 April to reports of a stabbing on High Street, Bromley.
“We sent resources to the scene including an ambulance crew, a paramedic in a fast response car, an incident response officer and dispatched a trauma team in a car from London’s Air Ambulance.
“Our first crews arrived on scene in less than four minutes.
“We treated a man for stab wounds and took him to a major trauma centre as a priority.”
