Police have released CCTV of the moment a gang wielding a baseball bat chased down a teenager before stabbing him in the head.
After one of the victim's friends called the emergency services, police found the 16-year-old with stab wounds to the head, concussed and missing a tooth.
The London Ambulance Service attended and took the injured teenager to a central London hospital where he underwent surgery for a three-inch wound.
The CCTV was taken from a nearby shop.
The suspects are described as black men and of slim build. One, carrying the baseball bat, was reported as wearing a dark hooded top with a reflective strip. He also wore light coloured trousers, white trainers and a large baseball cap.
Another suspect wore a dark grey top, trousers and black trainers with white soles, while a third wore a black coat over a grey hooded top, also with a reflective strip on the hood. He wore grey jogging trousers and carried a black bag.
A fourth man who did not appear in the CCTV wore a maroon jacket, black trousers and white trainers.
