At around 8pm on Friday, December 18, a 42-year-old woman was walking on Regent Street when a man approached her.
The male grabbed her and attempted to drag her into an alleyway towards Portland Street and Lindow Square.
The woman was pushed to the floor and kicked by the man before he ran back down the passageway.
The male is described as being of large built, around 5ft 9in tall, dressed in black with a clumsy run.
Detectives are appealing to anyone who was in the area at the time of the incident to contact police.
Det Con Trevor Walker of Lancaster CID said: “Incidents such as this are extremely rare but this was clearly very distressing for the female involved.
“I’m appealing for anybody who saw anyone in the area, hanging around or running away, or heard this lady shouting for help, to contact us immediately.”
Anyone with information can contact police on 101 quoting crime reference WB1510475.