Crews were called to the single-storey building at Bournville Road, Blaina, Abertillery, at about 03.30 GMT.
A 650ft (200m) cordon has been put in place and the road is shut. Residents have been told to leave their homes because there are gas cylinders inside.
The fire service said most people had gone to relatives' homes but some have gone to a nearby leisure centre.
Gwent Police and Western Power Distribution staff are at the scene.
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About 25 homes have been evacuated as firefighters tackle a fire at an MoT garage in Blaenau Gwent.