A 21-year-old woman was raped in Brighton by a man she met earlier in the evening, according to police.
Detectives said the woman from the Moulsecoomb area of the city was attacked in Temple Gardens at about 0220 GMT on Tuesday.
The attacker, who is described as white and aged about 18 or 19, fled the area after a resident said they would call the police.
Officers have been carrying out a forensic examination of the area.
The force has also been conducting house-to-house inquiries.
Det Insp Steve Warner, of Sussex Police, said: "Officers are offering continued support to the victim during this difficult time and are conducting a meticulous investigation into this serious offence.
"We are asking the local people of Brighton for their support while we investigate this rare and isolated incident.
"I am keen to speak to anyone who may hold information pertinent to this investigation. However insignificant the information may seem, please contact us immediately."
The suspect is said to be 5ft 7in tall, skinny build and with dark brown spiked hair. He was wearing baggy jeans and a baggy top.