A 17-year-old boy has died after being stabbed outside a tube station in south west London.
Police were called shortly before 5pm to reports of a stabbing outside Clapham South tube station.
The victim was taken to hospital but died less than an hour later.
Scotland Yard said no arrests have been made and there are closures in places on surrounding roads and the tube station.
Robin Pritchard, who witnessed the aftermath of the stabbing, said he moved through a crowd and saw the victim lying in a pool of blood while someone provided CPR.
The 47-year-old said: "I saw the guy on the floor, a pool of blood like he'd been stabbed in the stomach, his clothes were soaked.
"It was strangely calm and quiet. I think everyone was in shock. Lots of kids there from the local college.
"All I really registered were grey tracksuit bottoms which were soaked with a pool of blood coming from his waist like he'd been stabbed in the stomach."
Another witness, who did not want to be named, said he saw "a bunch of guys" shouting and appearing agitated, with one carrying a large knife.
He added that the area is usually noisy after students leave a nearby college.
He said: "I had no idea it was different to any other time the students get a bit noisy, until I saw the knife as I was literally passing them."
A Metropolitan Police statement said: "Police were called at approximately 16:35 on Friday November 2 to reports of a stabbing outside Clapham South Tube Station.
"Officers and London Ambulance Service attended.
"A 17-year-old male was found suffering from a stab wound. He was taken to a south London hospital where his life was pronounced extinct at 17:23.
"His next of kin have not been informed.
"Formal identification has not yet taken place and a post-mortem examination will be held in due course.
"Police remain on scene. No arrests. Enquiries ongoing.
"Road closures are in place and Clapham South Station has been closed."
The killing marks the 115th homicide in the capital so far this year.
Earlier on Friday, police confirmed a 15-year-old boy had been fatally stabbed in southeast London.
Officers were called at around 5.20pm on Thursday to Randlesdown Road, Bellingham, to reports of a stabbing.
The victim was taken to a south London hospital with stab wounds and was pronounced dead at 8.15pm, the Metropolitan Police said.
No arrests have been made and officers are working to establish the circumstances of the death.
The victim's next of kin have been informed, and anyone with information is asked to call police on 101.