A 15-year-old boy was stabbed to death after picking up his younger sister from his old primary school in Hackney.
Emergency services attempted to save the teenager after he staggered about 50 metres at Benthal Primary School before collapsing on Stellman Close at around 4pm on Tuesday.
However, he was pronounced dead at the scene and is now the second 15-year-old to die this week, following the fatal shooting of Rene Graham during a family day at a park in Ladbroke Grove on Sunday.
Detective Chief Inspector James Conway, head of Hackney police, said on Wednesday that a 15-year-old boy had been arrested on suspicion of murder. "The family are devastated," he added.
Witnesses spoke on Wednesday of their shock at the latest killing, which took place on the last day of school. One local said: "The boy used to go to the school. It's an absolute tragedy. He came here on an innocent visit and ended up dead."
A youth worker added: "The boy was at his old primary school. After he was stabbed the receptionist came out and said, 'I hope it's not him'. But it was and everyone was so upset. I heard that after he went to school he was going to meet a girl. Then he was attacked, it was a set up. They demanded his phone, he refused and he was stabbed in the back."
After the incident, she said the boy's grandmother and uncle arrived at the scene. The witness added: "They were devastated. I also saw one of the policewomen with a tear in her eye. There was a lot of sadness."
Tom Wiecielkowski, 43, a driver who lives next to the crime scene, described the "horrific" moment the boy collapsed.
He told the Standard: "The boy was injured and had staggered to the end of the lock, where he collapsed. Neighbours rushed outside with pillows and towels to help him. Then paramedics arrived and worked for hours to save him.
"They were giving him chest compressions and I could see he was still breathing. It was strange that I didn't see any blood, but they were more concerned about an injury to his side. He didn't say anything and the way he looked, I feared the worst. It's hard to get those images out of your head."
Two Lime rental bikes have been cordoned off with police tape as detectives try to determine if they were used by a potential attacker.
Tariq Shakir 41, a bus driver, ran outside with neighbors to help the victim of the latest knife attack.
He said: "We rushed outside when we saw the boy had collapsed. He looked badly injured and was very pale, honestly he looked like he was dying. His eyes were closed. Some neighbours used a towel to stop the bleeding before the emergency services arrived.
"The medics were amazing, they worked for three or four hours. They just didn't give up. It gives you faith in human nature at such a terrible time. They were heroes."
He added: "We've had problems here with drug dealing and one time a bullet went through my window in crossfire. Still, a life lost takes it to another level."
Detective Chief Inspector Kelly Allen, who led the investigation, said: "We know this happened in a busy area during the day and there would have been people in the area. It is believed the suspect ran away from the scene and I would urge anyone who saw anything suspicious in the Stellman Close area around 4pm to contact police."
Detective Chief Superintendent James Conway added earlier: "The death of a child is devastating and my first thoughts are with his family and friends. Our communities will also be deeply concerned by this event. Urgent enquiries are ongoing and my officers are working hard to identify the suspects.
"Residents can expect a significant police presence in the area as investigations continue and there will be road closures in the Stellman Close area for some time. Our communities have shown their strength by coming forward to support the police in tackling knife crime and I once again ask for their support."
The murder in Hackney follows the fatal shooting of Rene Graham in Emslie Pleasance Park in Ladbroke Grove on Sunday night. Witnesses said they believed the killing was a targeted "assassination" and the result of a conflict between rival gangs in the area.
The killer is believed to have jumped from a bush, fired into the air to avoid the crowd and then shot the teenager, who was pronounced dead at the scene. Four men arrested on suspicion of murder were later released on bail and police are appealing for information as they continue their search for the killer. Residents yesterday warned of a "summer of bloodshed" amid fears of reprisals.
Kensington MP Joe Powell said during PMQs on Wednesday: "My condolences go out to his family and the wider community who are feeling anxious, scared and shocked. Can the Prime Minister ensure that North Kensington receives support from this Government at this difficult time?"
Sir Keir Starmer described the deaths as "shocking".
"The loss of a teenage boy in West London is shocking and our thoughts are with his family and friends. I urge the public to support the Metropolitan Police with any information that may assist their ongoing investigation.
"Making streets safer is one of the five core missions of this government and this is a shocking reminder of how important that mission is. We have the ambition to reduce this kind of violence in our communities."
In another recent incident, another 15-year-old boy was lucky to survive after he was shot on Sumner Road in Southwark shortly after 11pm on Monday night, police said.
The Met said the attack was "an attempted murder of a 15-year-old boy" and it was "pure luck" that there was no murder investigation underway.
Anyone with information is asked to call 101 and quote CAD 5387/23Jul, or Crime fighters anonymously on 0800 555 111.