A local resident in flats overlooking the murder scene said he was surprised that the stabbing happened on his doorstep in what he thought was a quiet and affluent area.
"I came outside just seconds after it happened because I heard a lot of commotion and the victim was shouting."
"I came outside and saw the victim standing and holding his stomach and about three or four young blokes running down the street opposite. He was reeling and swaying for about 20 seconds and then fell and then people started gathering and calling the ambulance."
He said emergency teams had worked for a "long time" at the scene in an effort to save his life.
"It was a pretty serious wound - you could see it from a long way off. It was horrible. Once everyone had left there was a huge puddle of blood."
Cllr Nickie Aiken, Westminster City Council’s portfolio holder for community protection, who is meeting the Metropolitan Commissioner Sir Bernard Hogan-Howe this afternoon when he visits a Westminster Council run anti-gang programme, said:
"Our thoughts are with the family of this teenager and we will offer whatever help we can.
“Westminster City Council is committed to keeping the streets safe – including tackling gang culture - and we will work over the coming days and weeks to reassure the community of that."