Residents said seven children have been rescued from the building that housed a school with about 100 pupils.
Workers on top of the rubble shoveled debris away as thousands of people swarmed around the rescue site — dozens watching from rooftops and hundreds more packed into the surrounding streets.
The building was in the Ita-faji area of Lagos Island, the original heart of the lagoon city before it expanded onto the mainland.
Nigeria is frequently hit by building collapses, with weak enforcement of regulations and poor construction materials often used.
(Production: May Ponzo)