Police and four fire crews from Cosham and Southsea fire stations were mobilised to the fire in the former Grosvenor Casino in Osborne Road at 3.55am this morning.
Video footage captured by a resident shows grey smoke pouring from a ground floor entrance to the unused site.
Firefighters entered the building to discover large stacks of paper alight.
The blaze was relatively small compared with the amount of smoke created, according to one firefighter at the scene.
The Southsea emergency worker said: ‘Four pumps attended, and we were there for about an hour and a half.
‘We believe the cause was nothing untoward.
‘There was more smoke than fire in the building.’
No one was injured in the incident and police attended the scene.