The blast on Friday morning was believed to have been caused by a massive gas leak, Athens mayor Haris Doukas said on Friday night.
He said neighbouring buildings were also damaged and were evacuated.
The fire department said on Saturday that a second person was found unconscious in the rubble of the two-storey house in Plaka, the historical part of Athens that lies at the foot of the Acropolis hill, Greece's most-visited ancient monument.
Earlier on Saturday, an unconscious woman was taken away by an ambulance.
Two other women were injured from an adjacent building on Lysikratous Street, fire brigades said.
The search and rescue involved 30 firefighters and three dogs.
An investigation into the blast was under way.
Video from the scene showed rubble strewn across the street, with parked cars smashed by falling debris.
The people rescued by the fire department from the adjacent building were all tourists, resident Andreas Martzaklis said.
The pastor of a nearby church said that Sunday's service would be held outdoors because of damage to the church.
