Police were called to Old Kent Road around 10.45pm last night (August 27).
Officers arrived with paramedics, who treated three people at the scene for stab wounds and took them to hospital.
A 17-year-old boy is in a life-threatening condition.
LIVE as major road closed overnight after triple stabbing in south east London
A second 17-year-old boy has less serious injuries that are not life-changing or threatening.
The Met has been contacted for information about the third patient.
Officers chased a 17-year-old boy who was arrested on suspicion of attempted murder.
He remains in police custody as officers appeal to the public for more information.
Three people taken to hospital after triple stabbing in south east London
Old Kent Road, part of the A2, remains closed to eastbound traffic and officers are asking people to avoid the area.
A Met Police spokesperson for the Met said: “Police were called at 22.47hrs on Wednesday, 27 August to reports of a stabbing on Old Kent Road.
“Two boys, both aged 17, were found at the scene with stab wounds. One remains in hospital in a life-threatening condition.
“The second was taken to hospital where his injuries were deemed not life-changing or threatening.
“Following a pursuit a 17-year-old boy was arrested by officers on suspicion of attempted murder. He remains in police custody.
“Enquiries continue. If you have any information that could help police, please call 101 quoting CAD 8387/27AUG. If you wish to remain anonymous, please speak with Crimestoppers on 0800 555 111.
“Old Kent Road eastbound remains closed at this stage. Please avoid the area where possible.
“We understand the concern this incident will cause within the local community.
“Residents can expect to see an increased police presence in the area over the coming days.”
A London Ambulance Service spokesperson said: “We were called at 10.44pm on Wednesday 27 August to reports of a stabbing on Old Kent Road, SE1.
“We sent resources to the scene, including ambulance crews, an advanced paramedic, a paramedic from our tactical response unit and an incident response officer.
“We also dispatched a trauma team from London’s Air Ambulance, which consists of a doctor and a paramedic in a car.
“We treated three patients for stab wounds and took them to major trauma centres.”
