A murder inquiry was underway on Tuesday after a 22-year-old man was stabbed to death on London's Oxford Street, one of the country's busiest shopping districts, police said.
The victim, who has not been named, was attacked outside a McDonald's near Oxford Circus underground station shortly before 5 p.m. on Monday.
Police said the man became embroiled in a fight with an unknown number of attackers. He was stabbed on the pavement in front of hundreds of shoppers.
Paramedics tried to revive him at the scene. He was taken to hospital, but later died.
Detectives from the Metropolitan Police's Homicide and Serious Crime Command have launched a murder inquiry.
They were continuing to interview witnesses in a bid to establish how the incident unfolded.
Detectives said they were keeping an open mind about the motive. No arrests have been made. A post-mortem examination is due to take place later.
One witness, who did not want to be named, told several newspapers: "I just heard the screaming and then saw a couple of black guys run past us."
Newspapers also published pictures of a bloodied four inch knife, believed to be the murder weapon, that had apparently been dumped about 50 yards away.
"At this very early stage it is believed the victim had exited McDonalds, and was then involved in an altercation on Oxford St following which it is believed he was stabbed," a Scotland Yard statement said.
Knife crime has blighted London in recent months - it was one of four serious stabbings in 48 hours in the capital.
Mayor of London Boris Johnson has made the fight against knife crime a priority in his first term.
London's Deputy Mayor for Young People, Ray Lewis, said the crime was a "rising onslaught".
"This isn't a blow-out, this is a slow leak," he told BBC television. "This has been an increasing and steadily rising onslaught of lawlessness and abandoned behaviour for a number of years."
Jimmy Mizen, 16, was slashed in the throat with glass after declining to become involved in a fight with a man, said by newspapers to be a 19 year-old Turkish man.
Another two teenage boys were stabbed in Holloway, North London on Sunday night while 16 year-old Jimmy Mizen was slashed in the throat.
Police said they were recovering in hospital. No arrests have been made over that incident.
