At its peak up to 40 firefighters were called to tackle the blaze at the B&M bargain store on Rosemary Street.
The shop was extensively damaged when flames ripped through the first floor of the three-storey property just after 5am.
Neighbouring outlets have also been badly smoke damaged.
A spokeswoman for the Northern Ireland Fire and Rescue Service (NIFRS) said it was too soon to say how it started.
"At this stage it's too early to determine the cause of the fire and an investigation will begin in due course," she said.
Ten fire appliances including specialist aerial units were called to the scene after police officers noticed the fire.
NIFRS said crews had worked in challenging conditions throughout the day to stop it spreading.
A support unit is still at the scene to dampen down hot spots.
"It is likely that firefighters will remain at the scene for the next few hours," added the spokeswoman. "The cordon has now been reviewed and members of the public and business owners can now access the arcade."
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