An 18-year-old man was stabbed to death in broad daylight on London's Oxford Street which was packed with shoppers for the start of the Boxing Day sales.
The victim died just outside the Foot Locker sports shop on Oxford Street, near the junction with Stratford Place.
He has been named as Seydou Diarrassouba of south London. The Metropolitan Police said next of kin had been informed
A 300-metre section of the shopping thoroughfare was barricaded on one of retail's busiest days of the year. Several shops closed.
Police and ambulance crews attended the scene.
Eleven people were arrested in connection with the incident and remain in custody at various London police stations.
One Twitter user shared a photo taken from within the police cordon that shows a flick knife on the ground.
"Someone just got stabbed in Oxford St outside Foot Locker, they're shutting the place down," another witness posted on Twitter.
Police said there was a second stabbing on Oxford Street five hours later. A 21-year-old man was injured in the thigh. Police could not confirm whether the two incidents were related.
The man stayed in hospital overnight but his condition has been described by police as "stable".
A police spokesman said despite the two attacks, the public should not feel unsafe shopping in the West End.
"There is a high police presence here, we have a number of operations in place already for the shopping times because we were aware of the amount of people who would be here," he said.
"We are going to go back and review the intelligence for both incidents and see if we need to increase the police presence but at the moment we are not concerned for ordinary members of the public who are shopping."
Oxford Street has now fully reopened to pedestrians, buses and other traffic.
Anyone who witnessed the incident or has information that may assist police should call the incident room on 020 8721 4906. To remain anonymous, call Crimestoppers on 0800 555 111.