A 17-year-old girl woke up to find a man raping her in the street.
The assault took place off Hythe Bridge Street in Oxford between 2:40 and 2:50 GMT Saturday.
The teenager would have met his attacker in the discotheque The Bridge before leaving with him.
Thames Valley police said she remembered being woken up and telling the man to stop, but he ignored it. She is supported by specially trained officers.
The striker is described as white, in his late teens or early twenties, about 5 feet 9 inches tall and of fine build.
Detective Inspector Jon Capps said: "This is clearly a very painful incident that has been extremely traumatic for the victim.
"Members of the public will likely see an increased police presence in Oxford while we investigate.
"We are doing everything in our power to locate the person responsible for this incident, but we need the help of the public."