He said the witness recalled how the boy was possibly unconscious when he was stabbed, with his helmet thrown up to 10 metres away.
The man added: “He thought it was an accident, that somebody had knocked someone, but then he said they quickly came (out of the car).
“They beat him up, and they stabbed him, and they left.”
Cabinet Office Minister David Lidington conceded he was terrified at the amount of knife crime on British streets, and told the BBC’s Today programme: “Your heart goes out to the parents and the family of that young lad. Nobody should have to live through their children being attacked, let alone murdered in this way.
“We need to do two things. We need to make sure we have the right laws in place to deter and to punish the unauthorised use and carrying of knives.
“The laws have been strengthened under us and under previous governments on this, but critically I think we have to really step up efforts – and these are difficult and complicated – to wean young people away from the temptation of getting sucked into some of these gang networks in the first place.”
Mayor of London Sadiq Khan tweeted his condolences, urging witnesses to come forward.