East Sussex Fire and Rescue Service were called at 11.10pm on 31 March to reports of a fire at a commercial property in Terminus Road, Eastbourne.
Six fire engines, an Aerial Ladder Platform, the Command Unit and a number of officers have been in attendance.
Firefighters are using breathing apparatus and two main jets to tackle the fire.
Sussex Police are also in attendance.
There are no casualties reported and inquiries are ongoing to determine the cause.
As of 2.12am (1 April) the incident was being scaled back to 3 fire engines.
As of 3.08am one hosereel jet was in use and crews were checking for any hot spots using a thermal imaging camera.
As of 5.43am all firefighting operations were complete but a reinspection is planned today to check the building again.
UPDATE from Sussex Police:
Police have arrested a man on suspicion of arson following a fire at a former nightclub in Eastbourne on Monday (31 March).
Emergency services were called to reports of a large fire on the top floor of a building in Terminus Road, Eastbourne at around 11.10pm.
East Sussex Fire and Rescue Service brought the fire under control, while police officers conducted enquiries around the circumstances.
A 24-year-old man from Eastbourne was arrested a short while later on suspicion of committing arson, and remains in custody at this time.
To report any information concerning this incident, contact police online or by calling 101 quoting reference 1454 of 31/01.