Police are appealing for anyone who saw a man being removed from the Havre des Pas festival by security staff to get in touch.
The force said the incident took place on Roseville Street, near Roseville Bistro, on Sunday 26 July.
Anyone who witnessed the incident or has further information is asked to contact the police on 612612.
Information can also be given anonymously to Crimestoppers on 0800 555 111 or via its online form at crimestoppers-uk.org.
