The incident occurred at the Corn Exchange about 11.30pm on Friday.
A 29-year-old woman was left with a significant injury to her face.
Officers are urging anyone with information that can assist them with their inquiries to please get in touch.
They are particularly keen to speak to two women who left the premises immediately after the incident.
The women are described as being white and in their early twenties. One is about 5ft 9in tall and slim-built and the other around 5ft 6in tall and of average build. Both were wearing distinctive clothing - black military style ‘Michael Jackson’ jackets, black hot pants, white knee length socks, black shoes and hats.
Detective Constable Joyce Gunderson said: “I am keen to hear from anybody who witnessed the assault or who saw these women.
“Similarly, I would also urge both women to come forward and assist with our ongoing inquiries.”
Anyone with information is asked to contact police on 101 or anonymously via Crimestoppers on 0800 555 111.