A teenage boy has died after being stabbed in north London, with police appealing to witnesses who may have seen the attack.
Officers were called to Highbury Fields, Islington, just before 9pm on Thursday, the Metropolitan Police said.
A 15-year-old was treated at the scene, but he died a short time later in hospital.
His identity has not been confirmed, although police said his next of kin have been informed and are being supported by specialist officers.
There have been no arrests and inquiries into the circumstances are continuing, Scotland Yard said as it launched a murder investigation, led by Detective Chief Inspector Joanna Yorke.
She said: "My team is working hard to piece together the schedule of events that led to this tragic loss of a young life, but we still need information from people who saw what happened.
"Were you there? Did you capture any footage? Do you know who was responsible for the attack on the victim?
"Even if your information seems insignificant, please do the right thing and share what you know. If you don't want to speak directly to police Crimestoppers, they won't even ask you for your name.
"A young life has been brutally ended and a family is struggling to come to terms with their loss - if you can help please don't hesitate."
Local MP Jeremy Corbyn said: "Devastating news of the fatal stabbing of a teenager in Highbury Fields yesterday. My thoughts are with their family at this incredibly difficult time."
Gang of 'around 70 youths' spotted at the scene
The cordon surrounding Highbury Fields, which covers the whole park, will be in place for "at least the rest of the day" and possibly for a couple of days, police at the scene said.
Albina, who lives locally and wished to be known only by her first name, said her daughter witnessed a gang of "around 70" youths approaching the park approximately an hour before the stabbing.
She said her daughter, a teacher, was walking with her 15-year-old son at about 7.45pm when she saw the group, which she guessed included children as young as 15.
Kimberley Patton-Bragg and Michael Bragg, visiting London from New Orleans, said they witnessed a police presence as a group of between 15 and 20 young people became "heated" at about 7pm.
Ms Patton-Bragg said: "Kids - it was definitely kids - were being rambunctious, and it was definitely getting heated."
The couple were sitting inside their AirBnB, which faces on to the park, with the windows open when they saw the situation develop.
Mr Bragg said: "You could feel it building, at some point the groups became larger."
Anyone with information is asked to contact police on 101 or tweet @MetCC and quote CAD 7948/4Aug. Alternatively, contact Crimestoppers anonymously on 0800 555 111.