A boy, believed to be 16, has died after being stabbed in central London, Scotland Yard has said.
Police were called to the Lupus Street area of Pimlico shortly before 7pm on Sunday to reports that a youth had been stabbed.
The boy was found by ambulance staff and taken to a south London hospital where he died a few hours later.
A murder investigation has now been launched by police.
A spokesman for the Metropolitan Police said there have been no arrests.
He said: "A murder inquiry is now under way and an incident room will open in due course under detectives from the Specialist Crime and Operations Directorate. "We must retain an open mind re the circumstances of the incident and any motive at this early stage."
The murder scene in Lupus Street remains cordoned off with a number of police present. The street, close to Pimlico Underground station, is bordered by extensive council estates on one side and is home to Pimlico Academy.
Mohammed Alzubaidi, 49, from nearby Peabody Close, said his friend had witnessed a gang made up of black and white teenagers carrying out the attack.
"He said they were carrying knives, some of them with wide swords. He said the victim was screaming and trying to get away," he said.
Anyone with any information can call police on 101 or contact Crimestoppers anonymously on 0800 555 111.