Officers found the doors blown off and smoke damage inside the building in Hotham Park, Upper Bognor Road, at 5pm.
No serious injuries have been reported.
Two men - aged 37 and 23 - were arrested at the scene on suspicion of arson and have been taken into custody for questioning, confirmed police.
The cause of the fire is currently under investigation.
A spokesman said: “Police are keen to speak to anyone who was in the area at the time or who saw anything suspicious to report online or call 101, quoting 1021 of 20/10.”