One person who suffered burns was taken to hospital, Giorgos Mathiopoulos, the head of workers at the National Centre for Emergency Care, told Skai TV.
The blast sparked a fire in the six floor building. Some 24 fire-fighters with nine vehicles tackled the blaze.
The building is on a major traffic artery of the city, filled with high-rise apartments and commercial buildings.
"At the time of the explosion I was exactly across from the building, it was strong, really strong," said taxi driver Christos Chalkias as he showed the cracked windows and windshield, as well as dents in his taxi, caused by the explosion.
It is not clear what caused the blast. Police are looking into a possible gas leak.