The accident late Monday (24) in the western town of Mahad led three disaster-response teams and sniffer dogs to work through the night, combing tin sheets, twisted metal and broken bricks.
Officials said many residents of the 47 flats inside the building were spared because they had already fled the town to escape the coronavirus pandemic.
The cause of the accident was not immediately clear but building collapses are common during India's June-September monsoon, with old and rickety structures buckling after days of non-stop rain.
National Disaster Response Force spokesman Sachidanand Gawde told reporters that locals and emergency workers had retrieved the bodies of 11 victims in addition to the little boy who survived the collapse.
Video of the rescue effort showed onlookers applauding and cheering as the child was plucked out of the wreckage and hauled up on a stretcher.
Estimates of the number trapped ranged from 20 to 70 on Tuesday morning after dozens managed to flee when the building began to shake.
"No one knows how many people are actually stuck inside," a Mahad police official told AFP on condition of anonymity, adding that authorities had initially feared much worse, with early estimates as high as 200.
Mahad legislator Bharat Gogawale said that many of the building's occupants appeared to have been out shopping when the accident occurred around 7pm.
Others had left the town altogether, preferring to wait out the pandemic in their home villages.
"Many families were not residing in the building as they went to their native places due to the coronavirus-induced lockdown," district official Nidhi Choudhari said.
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