At around 8.30am on Wednesday May 10, a 16-year-old girl was walking to school on Salmons Brook, Edmonton, towards the A10.
As she approached the footbridge near Streamside Close, she noticed a man on the floor pleading for help.
The girl went to help but the man grabbed her, then pushed her to the floor and tore her shirt – he also threatened her with a knife, Scotland Yard said.
The man then ran towards Cedars Court.
The girl returned home and the police were called – she was not injured, she is accompanied by specially trained employees.
The suspect is described as a white male in his 40s to 50s, about 6’2″ tall with a stocky build.
He spoke with a London accent and had no facial hair.
He was wearing a black and gray beanie that covered his hair, a dark green tracksuit with a dark green coat over it, a black t-shirt and dark woolen gloves that looked worn.
Det Con Darren Humphreys, from the Northern Area Command, said: “I am appealing to all pedestrians who used Salmons Brook on the morning of May 10 and witnessed this incident to come forward.
“This was an extremely distressing attack for this young girl. We continue to support her, but we must ensure that the person responsible is arrested and taken off the streets.”
No arrests were made, investigations are ongoing.
Anyone with information that could help police should call 101 or tweet @MetCC quoting CAD2221/10May.
People can also contact Crimestoppers anonymously on 0800 555 111.