"There was a loud sound like a cracker and we heard yelling, that's when we rushed out," said Garima Niwaz, a 22-year-old housewife who stood by the site cradling her young son. "No one was sure what had happened for some minutes till we saw the back portion of the house had fallen in," she said.
At least 21 other people were injured in the accident, police said.
"There could be more people trapped under the debris," said police officer Dheeraj Chauhan, adding that torrential rains were hampering rescue efforts.
Police said 11 people died in the wreckage while nine others who were critically injured died later in hospitals.
Heavy monsoon rainfall and shoddy building construction are often blamed for building collapses in Mumbai.
Police estimated that the collapsed building was over 60 years old.
