A 21-year-old woman, who did not want to be named, said the teenager was "just chilling with her friends" when she was shot from a auto for "no reason at all". "I even tried to save her - had to, had to".
She said the gunshot wound, below the victim's breast, was not immediately visible and it looked like she was "having a fit".
He said: "It was a awful attack in which this young lady has lost her life".
Scotland Yard said the 15-year-old boy who was stabbed in Walthamstow has life-changing but not life-threatening injuries.
"I can't believe she's gone". She didn't know what to do.
There was a poignant silence when the body of the 17-year-old was taken away, broken only by the sound of people crying. "Her mum didn't deserve to watch her die".
She told how the girl's mother arrived before paramedics, adding: "She was screaming".
'I heard the bangs because I sleep in the front room, ' she said.
Including January's figures, there have still been more murders in NY, which has a similar-sized population to London, but British politicians and police are increasingly expressing concern about the higher United Kingdom numbers, driven by a surge in knife crime.
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According to the latest figures, some 23 people were killed in London over the past month in comparison to 21 violent deaths in NY. The number of killings reached a peak around June before dropping again in the second half of the year. "One murder is one too many, and we are working hard with our partners to understand the increase and what we can do to prevent these tragedies from happening in the first place", a Metropolitan Police spokesperson said.
The latest victim, a 20-year-old man, was stabbed to death after leaving a bar in the Earlsfield area of south west London in the early hours of Easter Sunday, according to The Daily Telegraph.
Croydon Central MP Sarah Jones, who chairs the UK's All-Party Parliamentary Group (APPG) on Knife Crime, told the BBC that London could learn from NY on how to reduce violent crime.
It comes as another grieving family warned "gun culture is becoming rampant in our community".
Jones added: "Knife crime and violent crime acts like an epidemic, so you need to go in at the source to cut it off and then you need to inoculate the future young people against it".
Prime Minister Theresa May's spokesman said on Tuesday: "there can be no place in our society for violent crime".
Proposed measures include a tightening of rules covering online sales of knives.
The recent spate of violence has prompted scrutiny of a sharp reduction in stop-and-search activity, with use of the powers at the lowest level since current data records started 17 years ago.