A 17-year-old girl woke up to find a man raping her in the street.
The assault happened off Hythe Bridge Street in Oxford, between 02:40 and 02:50 GMT on Saturday.
The teenager is believed to have met her attacker in The Bridge nightclub before leaving with him.
Thames Valley Police said she remembered waking up and telling the man to stop but he ignored her. She is being supported by specially trained officers.
The attacker is described as white, in his late teens or early 20s, about 5ft 9in tall and of slim build.
Det Insp Jon Capps said: "Clearly, this is a very distressing incident which has been extremely traumatic for the victim.
"Members of the public are likely to see an increased police presence in Oxford while we carry out inquiries.
"We are doing all that we can to locate the man responsible for this incident, but we need the public's help."