The 22-year-old suffered a stab wound to the chest
A man stabbed to death outside a fast-food restaurant in London's Oxford Street was attacked in a row after a drink was thrown, police have said.
Steven Bigby, 22, was stabbed soon after leaving a McDonald's restaurant with friends on Monday afternoon.
A knife was recovered from the scene and police say indications show he died from a single stab wound to the chest.
Officers said two groups of men clashed outside the restaurant in a "spontaneous incident".
Four men, who were black and aged between 20 and 30, fled the scene after the attack.
One of the group was wearing a red T-shirt, another was wearing a white T-shirt and a third was in a vest while the fourth was dressed in dark clothing, acting Dep Ch Insp Bob Mahoney said.
Police are also looking at CCTV footage and talking to eyewitnesses.
No-one has been arrested over the incident.
Appeal for photographs
Mr Mahoney said there were three men in Mr Bigby's group.
He said: "A drink was thrown. The incident escalated very quickly and Steven was fatally stabbed.
"At the moment we are treating it as a spontaneous incident. There is nothing to suggest that this is a gang-related crime or linked to any other incidents.
"There is a possibility the four men were walking up and down on Oxford Street during the afternoon. It is possible they may have been seen promenading."
The knife found at the scene is being subjected to tests.
Mr Mahoney appealed to members of the public to hand in pictures of the incident.
"I know that a lot of people took photographs of the incident. If you've got photographs taken in Oxford Street yesterday, I'm asking you to contact police urgently."
About half of the length of Oxford Street - between Oxford Circus and Tottenham Court Road - remains closed. Nine bus routes have been diverted.
The New West End Company, which represents the interests of businesses in the area, said in a statement: "Our sympathies are with the victim's family and anyone affected by yesterday's incident.
"We call on retailers and shoppers in the area to pass on any information they believe will be helpful to police investigations."