A 17-year-old boy has been stabbed to death outside a busy south-west London tube station, police have said.

Police officers were called to reports of a stabbing near Clapham South station shortly after 4.30pm on Friday. The teenager, who has not been named, was pronounced dead in hospital less than an hour later.

Robin Pritchard said he moved through a crowd and saw the victim lying in a pool of blood while someone provided CPR. He said: "It was strangely calm and quiet. I think everyone was in shock. Lots of kids there from the local college."

Another witness who did not want to be named said he saw a man carrying a large knife among a "bunch of guys" who were shouting and appeared agitated.

He added that the area was usually noisy after students left nearby St Francis Xavier 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."

Scotland Yard said: "His next of kin have not been informed. Formal identification has not yet taken place and a postmortem examination will be held in due course." Officers were still on the scene on Friday evening, but had made no arrests.

The killing is the 115th homicide in the capital so far this year and comes the day after a 15-year-old was stabbed to death in Bellingham, in south-east London.

Police identified the victim in that incident as Jay Hughes on Friday evening and appealed for information about his death.

Det Ch Insp Chris Soole, who is leading the investigation, said: "We know that this was an awful premeditated attack on a young boy who sadly lost his life as a result. I would appeal to anyone with information to get in touch with us. Those responsible must be caught and brought to justice. Any information passed to police will be treated with the strictest confidence."