Officers were called to the Devonshire Bar and Lounge, Devonshire Square, shortly before 11.30pm on Friday (July 31).
Two men were reported to have been involved in an altercation outside the venue, during which one of them suffered a facial injury.
Police investigating the matter are keen to hear from anyone who witnessed the incident, or who may have any other information about what happened.
Anyone wishing to get in touch can do so online or by calling police on 101 quoting serial 1905 of 31/07.
Alternatively, you can contact the independent charity Crimestoppers anonymously on 0800 555 111.