A teenager stabbed to death yesterday in the Boxing Day sales has been named by a school friend and described as a "big personality" who always had a smile on his face.

Horrified shoppers saw the 18-year-old stabbed to death in broad daylight during a fight at a Foot Locker sports shoe store on London's Oxford Street. Just hours later another youth was stabbed in the leg on the same busy street.

School friend Munawar Shaikh, 20, today said the dead man was Seydou Diarrassouba and lived on Phipps Bridge Road in Mitcham, south London. The pair went to Rutlish High School in Merton, south London, together and grew up in the same area.

Mr Shaikh, who now studies broadcast journalism at Leeds University, said: "I heard about it on social network sites. It's devastating, I had lost contact with Seydou but he was such a big personality and a person everyone in Mitcham would know.

"He was just a good kid who always had a smile on his face. I have not had a chance to go out yet but I imagine everybody will be cut up about it."

Asked if he thought the killing may have been gang-related, Mr Shaikh said: "I cannot say if it was a gang thing, it seems more like it was just a misunderstanding between youths."

This morning police appealed for witnesses with mobile phone footage of the stabbing to come forward.

Speaking outside Scotland Yard, Detective Superintendent John Sweeney said: "There were several people who witnessed this event, both inside and outside the store.

"A number of them recorded it on camera phones, and we're particularly interested in those people coming forward so we have access to that footage.

"But also anyone else who saw what took place inside and outside the store, we want them to come forward and assist us in establishing exactly what took place."

He urged witnesses to contact police at the incident room on 020 8721 4906 or Crimestoppers on 0800 555 111.

Scotland Yard said they had recovered a number of weapons from the first crime scene and arrested 11 people after the fight broke out at about 1.45pm.

Those detained included five 19-year-olds, three 17-year-olds, two 22-year-olds and one 16-year-old. They are in custody at several London police stations.

Three men were also arrested in connection with the second stabbing, which happened near Oxford Circus underground station at about 6.20pm.

A Facebook page called R.I.P. Seydou was set up for friends to pay tribute to the dead teenager, who was also known by the nickname "Nutz".

One message, posted by Okoko Felly, read: "my heart gose to ur mum and dad bruv, whats your big sis an ali gonna do with out u, man why dose life ave to be lyk dis. forever in our hearts nutz i luv u boi. only 18".

Justin Osei wrote: "R.I.P man its not gonna be the same not seeing you around you were almost like a monument to mitcham, sad times".

And Kanyin Ifebogun said: "R.I.P Seydou, just the other day I was talking to you. Known from when we were little, wish you perfect peace in paradise. Gone but never forgotten hun. Will always remember our lil dancing moments in primary school and you always giving me joke. Sleep tight hunny".

Amateur video footage posted on YouTube showed angry scenes outside the Foot Locker store as police held back remonstrating youths while paramedics attempted to revive the first stabbing victim.

The video appears to show a policeman using a taser electroshock weapon on one of the youngsters, who can be seen exchanging shoves with officers. A Scotland Yard spokesman confirmed that an officer had discharged a taser.

After police shut off the first crime scene, Boxing Day bargain hunters and tourists were going up to the cordon to take pictures and ask officers what had happened.

Shoppers could be seen going in and out of nearby shops, such as Debenhams and Zara, but a number of stores were forced to close.

Inspector Bruce Middlemiss, the duty officer for Westminster Police, said last night that shoppers should not feel unsafe but that police would conduct a "review".

"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.

"We are going to go back and review the intelligence for both incidents and see if we need to increase the police presence here but at the moment we are not concerned for ordinary members of the public who are shopping."

Richard Dickinson, chief executive of the New West End Company, which represents retailers in Oxford Street, Regent Street and Bond Street, said the organisation was "shocked" by the two stabbings but claimed such incidents were "extremely rare" in the area.

He added: "New West End Company is seeking rapid reassurance that the Metropolitan Police is adequately resourced for such events and have called for an early meeting with Deputy Mayor for Policing Kit Malthouse and senior operational Commanders to ensure such incidences are learnt from and not repeated."

Mr Dickinson said all shops had opened as normal on Oxford Street this morning except for Foot Locker, where there was still a "small crime scene".

The company reported £15m sales in the first three hours of the Boxing Day sales despite a drivers' strike on the London Underground.