Scotland Yard appealed for witnesses to the assault on Sunday in which the boy - who boasted that he was 11 years old - dragged the girl into an alley off Villiers Road saying he had a knife and threatening to kill her.
He tore the girl's clothes off before assaulting her.
The girl's ordeal lasted 15 minutes and she said her attacker also followed her part of the way home before disappearing.
Police are looking for a black male suspect aged between 11 and 15.
'He engaged his victim in conversation before dragging her into the alleyway where he raped her,' a police spokesman said. Although the girl was terrified after her ordeal, she did not require hospital treatment. The assault happened between 5.30pm and 6pm as the girl walked home from a chip shop. Her attacker is described as being 5ft 3in tall, of slim build, with a small scar on his cheek.
'He had short, dark hair with some sort of pattern or razor-cut at the back, and he wore a light blue polo shirt, faded blue jeans and a black waist-length jacket with yellow sleeves,' the spokesman added.
Police are also trying to trace a white woman and a black man whom the girl saw walking towards nearby Dudden Hill Lane at the time of the attack.
Anyone with any information is asked to contact Acting Detective Inspector Stephen Condon at Kilburn CID on 081-733 3737.Reuse content