A teenager was reportedly heard screaming for his life before he was killed by a gang brandishing knives and swords.
The boy, believed to be 16, died after he was stabbed in Pimlico, central London, shortly before 7pm on Sunday.
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.
He said his friend told him the gang had scattered and run off in different directions.
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.
The boy’s mother was seen “screaming with grief” as she pleaded with police to let her through the cordon as her son lay bleeding, the Evening Standard reported.
Eyewitness Steve Russell told the newspaper: “She was screaming with grief and shouting her son’s name but they wouldn’t let her through.“
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."
There has been no arrest.
The murder scene in Lupus Street remained cordoned off today 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 58-year-old local resident, who did not want to be named, said there had been two murders in nearby Gloucester Street in recent years and a stabbing of a boy outside a row of shops in Lupus Street before Christmas.
"The violence that has started to occur around here is something else, it is frightening for all the residents," she said.
"You used to be able to walk around at night but I am starting to be very wary, especially at night-time."
Nickie Aiken, Westminster City Council's portfolio holder for community protection, said the murder would be "top of the agenda" at a meeting with Metropolitan Police Commissioner Sir Bernard Hogan-Howe this afternoon.
She said: "Once more we have seen the tragic blight that knife culture has inflicted on the capital.
"Our thoughts are with the family of this teenager and we will offer whatever help we can.
"Westminster City Council is committed to keeping the streets safe - including tackling gang culture - and we will work over the coming days and weeks to reassure the community of that."
A 22-year-old resident of a block of flats overlooking the murder scene, who asked not to be named, said: "I came outside just seconds after it happened because I heard a lot of commotion and the victim was shouting.
"I came outside and saw the victim standing and holding his stomach and about three or four young blokes running down the street opposite.
"He was reeling and swaying for about 20 seconds and then fell and then people started gathering and calling the ambulance."
He said emergency teams had worked for a "long time" at the scene in an effort to save his life.
"It was a pretty serious wound - you could see it from a long way off. It was horrible. Once everyone had left there was a huge puddle of blood.
"I am surprised that this has happened right on my front doorstep. I always had the impression that Pimlico was a quiet and affluent area."
He said the victim was white and the group he saw fleeing the scene were also white.
Anyone with any information can call police on 101 or contact Crimestoppers anonymously on 0800 555 111.