The girl, described as a young teenager, was attacked by an “unknown male” at around 11.30pm on Saturday.
She had left the party because she thought her mum was outside, but police say that was not the case.
Following the attack, in Warlingham, the girl was taken home by two passers-by, a man and a woman.
She is now being supported by specialist police officers following the “horrendous ordeal”.
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