The girl was in Walthamstow between 12.15am and 12.30am on Saturday when she met three men, police said.
She was taken to a garage area at the end of Hibbert Road, near Theydon Street, where she was pinned down and raped by two of the men.
The girl managed to call a friend who came to the scene and they left together. The suspects then left the area.
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Officers were given the touching letter at Brixton Station while they were on a reassurance patrol after yesterday's attack
The 52-year-old had previous convictions for violence but was not on the anti-terror watchlist.
In a statement Charlton Athletic said: Keith was a true hero who will be greatly missed by all the Charlton family and everyone at the club.