A teenager was apparently heard screaming for his life before he was killed by a gang brandishing knives and wide swords, it has been claimed.
The boy, believed to be 16, died after he was stabbed in Pimlico, central London, shortly before 7pm on Sunday night.
Mohammed Alzubaidi, 49, from nearby Peabody Close, said his friend witnessed a gang 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," Mr Alzubaidi said.
The fatally injured teenager was found by ambulance staff after emergency services were called to the Lupus Street area following reports that a youth had been stabbed. He was taken to a south London hospital where he died a few hours later.
A spokesman for the Metropolitan Police 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.
A 22-year-old resident of a block of flats overlooking the murder scene, who asked not to be named, said a woman believed to be the victim's mother had appeared at the scene after the emergency services arrived.
"She was wailing. She was in shock - she was shaking and trying to control herself. She wasn't allowed in, obviously," he said.