A man with a knife threatened a woman as she walked through Pavilion Gardens in the centre of Brighton.
He propositioned the 23-year-old woman at about 3.15am on Saturday (18 December) but she managed to escape.
The man is described as white, about 5ft 8in tall, with broad shoulders and of a large build.
He was aged in his late twenties to early thirties, clean shaven and wearing a white knitted cap with a peak and white knitted jumper with buttons at the top.
Detective Constable Chris Bishop, of Brighton and Hove CID, said: “We would particularly like to speak with a man who assisted the woman shortly after the incident by flagging down a taxi.”