Doorbell-cam footage shows her move towards the suspect before tasering him while he still brandishes the weapon, then armed colleagues handcuff the 36-year-old.
The boy suffered a catastrophic head wound in what police described as a "nontargeted" attack.
A man in a yellow hoodie had climbed onto garage roofs to evade capture in Hainault, East London, after the 7am horror yesterday.
Two police officers and two members of the public were also injured. Police said that a man was arrested on suspicion of murder 22 minutes after the first 999 call.
Chief Supt Stuart Bell called the attack "horrific", and added the boy's family was being supported by specialists.
He added: "Everyone at the Met is keeping them in their thoughts at this unimaginably difficult time."
The teenager had only just stepped out of his home to walk to school when James Fernando witnessed him being "stabbed in the head".
He said the suspect had asked one of his female neighbours to "take the telephone from him to tell whoever was on the phone his location".
Mr Fernando, 39, said: "Within two seconds after that she's realised something isn't right...started running and he's pulled a samurai sword from the back of his trousers.
"She's shouted to the other neighbour - a boy who was on his way to school. As he's turned around, he's struck him on the face... he was dead on the spot.
"It's quite traumatising now. I can't stop envisioning the boy's face." The youngster was taken to hospital but could not be saved.
The two wounded officers had emergency surgery and were last night nursing "significant" but not life-threatening injuries, while the two members of the public were recuperating in hospital.
Police who had raced to the scene were attacked during a 15-minute chase in which a suspect clambered on to garage roofs and marauded through gardens.
This story is from the May 01, 2024 edition of Daily Express.
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This story is from the May 01, 2024 edition of Daily Express.
Start your 7-day Magzter GOLD free trial to access thousands of curated premium stories, and 8,500+ magazines and newspapers.
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