Ruby Residency, being constructed by Bharat Realtors and Developers, crashed at around 3 pm.
Eyewitnesses said the structure, coming up on a busy road, crashed like a pack of cards. In view of the slow pace of rescue work, the state government sought help from the Army.
Two Army platoons are taking part in the rescue operation now.
Chief Minister Manohar Parrikar told reporters late in the evening that so far 23 people had been extricated from the rubble and among them 13 were dead.
"Builder has told the police that more than 40 workers were on the site when the structure crumbled," he said.
DIG O P Mishra said police have registered complaint against the builder, the contractor and others concerned with construction.
Parrikar was in the vicinity of the town (which is about 70 km from Panaji) to attend a state-sponsored folklore festival, and rushed to the site when the news came. The festival was cancelled.
Bodies were sent to a state-run hospital in Margao town. Medical aid is being provided in Cancona.
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