Dozens are injured and many others are feared trapped beneath the seven-storey building under construction in Thane, Indian media report, quoting officials.
Indian police said the building was being constructed illegally and four floors were already occupied.
Building collapses are common in India. Poor construction material and practices are often blamed.
A growing population and a lack of space has seen an increase in the number of high rises, but many builders fail to take sufficient safety precautions, or obtain proper permission to build.
A section of the building collapsed on Thursday evening, bringing the entire structure down, a police official in Thane, Maharashtra state, said.
He said at least nine children were among the dead.
Rescue efforts continued through the night.
The disaster management officials in Thane district said 57 people were injured.
Most of the victims were construction workers who were staying in the building, he added.
In December 2013 at least 13 people died when part of a half-constructed building collapsed in the Wagholi area of Maharashtra.
Last September at least six people were killed when a building collapsed in Pune city in Maharashtra.
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