A 15-year-old boy has died after being stabbed in west London during the Friday morning school run.
Officers were called to reports of a fight close to Global Academy on Blyth Road, Hayes around 08:30 BST.
The victim was found suffering from multiple stab wounds and died at the scene.
Formal identification of the victim has yet to take place and a post-mortem examination will be held in due course, police said.
The incident happened close to Global Academy at a time when youngsters would have been making their way to school.
The Met Police said a suspect had been detained, but provided no further details of what happened outside the college for students aged 14 to 19.
Tia Rek-Williams, 21, a teacher who lives nearby, said although she had not seen the stabbing, it was still "traumatising".
"They were saying 'come on, you've just got 30 seconds, hold on, keep fighting'," she said.
Ms Rek-Williams said six people had tried to help the boy before emergency services arrived and treated him with a defibrillator.
"It happened in broad daylight, a 15-year-old boy. This happened literally in the middle of the street, not down a back alley, in front of people's houses. It's just awful."
Chief Supt Peter Gardner, said: "I am deeply saddened to hear of the tragic loss of a child's life to knife crime in Hillingdon.
"A family has been devastated and the effects of this terrible incident will reverberate throughout our communities.
"As a parent of a teenage boy myself, I cannot imagine the pain the family is going through and I offer our full condolences and support."
He added that police patrols in the area will be stepped up.
Labour MP for Hayes and Harlington John McDonnell tweeted: "If anyone can assist the police with any information on this tragic incident, I urge them to come forward."
Cordons remain in place at the scene.