A 14-year-old boy riding a moped who was mown down by a car and then stabbed multiple times died in a targeted attack, police have said.
The teenager was murdered in an area notorious for drug dealing and blighted by so-called county lines gangs that are known to exploit children.
Chief Superintendent Richard Tucker said the age of the victim "beggars belief" as a section 60 order was put in place to allow officers to search anyone in the vicinity of the scene for weapons.
In 2018, around a fifth (17%) of homicide victims in London were teenagers, most of whom were stabbed. The youngest were 15.
The boy was rammed off the moped by a black Mercedes at around 6.30pm on Tuesday in Bickley Road, Leyton, east London, and was then stabbed several times by three attackers as he lay unconscious in the road.
Detective Chief Inspector Larry Smith, from the Metropolitan Police murder squad, said: "Everything that we have learned about this attack so far indicates it was targeted and intent on lethal force from the outset.
"We are doing everything we can to catch those who carried out this cowardly attack and bring them to justice."
Anthony Anderson, a mechanic who works near the scene, described the victim as "a very nice guy" whom he had tried to persuade to go to school and stay off the street.
Locals also said there were moped gangs of young people in the area and that drug dealing regularly took place.
Ali Yamah, a tyre fitter who has worked opposite Bickley Road for 17 years, said the area could be intimidating at night.
The 48-year-old said: "In my experience here there is drug dealing, this is the main source. In the evening, now it's winter time, they put on masks."
He added: "Sometimes we are afraid, people are afraid of this kind of behaviour."
Mr Anderson said he would often give the victim "fatherly advice" and thought he was older than 14 because he had a mature attitude.
He last saw the teenager, who would sometimes come to get his bike fixed, walking down the alley last week.
Speaking outside the garage where he works, he said: "He's somebody I really spoke to a lot, tried to get him off the street, tried to just get him to go to school ... it really, really hit me when I heard last night."
One man, who did not want to be named, described how a witness to the incident came into the premises where he works nearby, clearly horrified by what they had seen.
He said the witness told him that the teenager who was on the moped was struck by the car, which was travelling in the opposite direction.
"They said they knocked him off the moped and four people got out of the car and they stabbed him seven times - there were unsurvivable wounds in his back," he said.
He added that the witness said the boy was possibly unconscious when he was stabbed, with his helmet thrown up to 10 metres away.
The man said: "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."
The murder took place in the borough of Waltham Forest, in north-east London, which has has been blighted by gang crime, with the local authority ploughing £3 million over the next four years into a prevention programme.
It commissioned a report, published in the summer of 2018, which found that rival county lines gangs were operating in the area and fighting over control of the drugs market.
Police patrols were stepped up in the area in the wake of the killing.
Mr Tucker said: "Everyone is shocked and appalled by this murder. The age of the victim, a boy aged 14, beggars belief and I completely understand the alarm and concern it has raised from those who live and work in the area.
"My officers will be out working to support investigators today and be there to talk to the public and listen to the concerns they have. Neighbourhood patrols will be stepped up in the area and locals can expect to see more police.
"I would urge anyone with information on the attack to contact police without delay. "
Investigators are trying to trace the black Mercedes B Class involved in the crash, that is believed to have extensive damage to the front.
The leader of Waltham Forest Council, Clare Coghill, called for those with information to come forward, warning that "to stay silent is to support murderers".
She said: "The death of a child is a loss that no parent or family should ever have to face. Our thoughts are with all of the victim's family and friends at this difficult time.
"This is a tragedy that will be felt right across our borough in all our communities. These senseless acts of violence must end.
"The murder of a 14-year-old boy is an appalling act and we must come together as a community to bring those responsible to justice.
"There will be members of our community who know, or suspect that they know, what happened to this child. You need to come forward; to stay silent is to support murderers.
"The police cannot tackle the violence on our streets alone. Everyone has a role to play in fighting crime and bringing those responsible to justice. Without information these murderers will be free to do this again."