The stabbing to death of an 18-year-old in an Oxford Street store during the Boxing Day sales is thought to have been gang-related, police claim.
The victim was a member of the ABM (All 'Bout Money) gang, whose members repeatedly clashed with rivals 031 Bloods, police said.
It is thought that the two gangs were using the sales as cover for shoplifting before violence erupted.
Thousands of shoppers had converged on London's West End to make the most of the bargains offered by the sales. Among them were members of rival gangs.
Seydou Diarrassouba was fatally stabbed in the heart in a branch of Foot Locker at lunchtime. He bled to death in front of a crowd of horrified onlookers.
Footage released on YouTube showed a crowd grappling with police as paramedics worked on Diarrassouba, who was on bail on robbery and assault charges.
Police have questioned 11 people over the killing and hundreds of hours of CCTV footage will be examined for evidence.
A second stabbing took place on the same street hours later, when a 21-year-old was stabbed in the leg. There was also a third clash on the same day.
A Facebook tribute page saw hundreds pay their respects to Diarrassouba, a practising Muslim from Mitcham in south London with the nickname "Nutz", but it was also targeted by racist diatribes and gang slogans.
"RIP Seydou. Still the news hasn't really registered in my mind yet...just can't believe something like this could happen to someone who just lives around the corner from me. I remember you as being short with chubby cheeks, a little boy who always used to make me laugh with your jokes," read one tribute.
Others referred to him as a "fallen soldier", and lamented another young man "falling victim to the the streets".
Police are appealing for anyone who may have filmed the incident and the moments afterwards to share their footage and help them piece together events.
See the footage of crowds clashing with police, as paramedics try to save Diarrassouba's life, below: