London Ambulance Service said they treated five people at the scene and took them to hospital – including the 14-year-old boy.
In an update this afternoon, Chief Superintendent Stuart Bell, who leads local policing, confirmed the teenage boy had died.
He said: “He was taken to hospital after being stabbed and sadly died a short while after.
“The child’s family are being supported firstly by my local officers and now with some specialist officers and everyone across the Met is keeping them in our thoughts at this unimaginably difficult time.”
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Chief superintendent Stuart Bell giving his update
He also confirmed two other members of the public were in hospital with injuries believed to be not life-threatening.
Along with them, two officers received “significant” but not life-threatening injuries and will undergo surgery, the police chief said.
Officers arrested the man 22 minutes after the first call went into emergency services.
Eyewitnesses described hearing shrieking and seeing someone receive CPR at the scene.
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Video showed a man wielding a large sword outside a number of residential properties.
Police added they did not think it was a targeted attack, or terror-related, and they were working to establish the circumstances of what took place.
As the press conference ended this afternoon Ch Supt Stuart Bell refused to respond to questions over whether the suspect had been previously arrested.
Chris Bates told Sky News he saw police apprehend a suspect wielding a “big samurai sword”.
He said that, after confronting the suspect, police screamed at them to drop the sword, before they Tasered him, and “piled on top of him”, arresting him.
Another local resident said she saw a body on the ground as she hid by her window, while a sword-wielding man shouted “do you believe in God?” outside her home.
The witness, who didn’t want to be named, said: “He was wielding his sword trying to attack the police but then they sprayed him and he ran away.
“He was shouting at the police ‘do you believe in God?’, also at the ambulance.
“We were very scared and trying to hide and not show ourselves through the window, because he was standing right next to our house and he could have seen us if he looked up.
“We were trying to hide but also at the same time taking video of him attacking the police, and of the body on the floor, so, yeah, we were very scared and we didn’t know what to do.”
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The King has asked to be kept informed of the incident and has sent his thoughts to those affected.
A Buckingham Palace spokesman said: “Following the horrific scenes in Hainault this morning, the King has asked to be kept fully informed as details of the incident become clearer.
“His thoughts and prayers are with all those affected – in particular, the family of the young victim who has lost his life – and he salutes the courage of the emergency services who helped contain the situation.”
Politicians were quick to offer their condolences after news emerged of the attack.
Prime Minister Rishi Sunak said: “This is a shocking incident. My thoughts are with those affected and their families.
“I’d like to thank the emergency services for their ongoing response, and pay tribute to the extraordinary bravery shown by police on the scene. Such violence has no place on our streets.”
Local MP, and Labour shadow minister, Wes Streeting said the whole community was “devastated” by news of the attack but “deserved answers”.
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Mayor of London Sadiq Khan praised the “everyday bravery” of the police officers who ran towards the sword-wielding man.
He added: “This attack is devastating and appalling. I’m sure I speak on behalf of the entire city when I say my thoughts and prayers are with this young child and his family.”