An 18-year-old man is fighting for life after being stabbed in a park in north-west London.
Police believe the teenager was attacked just after 6pm in Montesole Playing Fields, in Pinner, on Wednesday.
Officers, paramedics and the London Air Ambulance attended and found him with a stab wound.
The teenager was taken to hospital, where he remains in a critical condition. His next of kin have been informed.
Detectives believe the victim was stabbed during a fight which saw another group of youths approach him and his friends.
Scotland Yard is appealing for witnesses, with no arrests yet made and a crime scene in place.
Detective Inspector Jonathan Newell said: “This is a very serious incident that has left a young man with life-threatening injuries and we urgently need information from the public.
“Anyone who was in the area of the park at the time may be able to assist. Did you see youths either going to or running away from the park?
“If you live near the park and have CCTV that might record people travelling there, let us know, we want to see that CCTV.
“Also if you live near the area and find anything in the street, or your garden, that might have been used or discarded in this assault, call us.
"Anyone calling police will be fully supported and you can always call Crimestoppers if you don’t want to give your name. But please, if you have information, make that call.”
Anyone with information is urged to call police on 101 or contact officers via Twitter @MetCC quoting CAD 5906 of August 26, ir call Crimestoppers anonymously on 0800 555 111.