During every monsoon season, which runs from June to September, India experiences fatal incidents of building and wall collapses as rainfall weakens the foundations of poorly-built structures.
Heavy rain brought a wall crashing down early on Tuesday morning on shanties built on a hill slope in Malad, a western suburb of Mumbai, a fire brigade official said. Eighteen people were killed in the incident.
"Rescue work is still going on," the official added. "So far we have rescued more than two dozen people."
Three people died when a school wall collapsed in the city of Kalyan, 42 kilometres north of Mumbai, late on Monday.
In the nearby western city of Pune, six people were killed in a wall collapse early on Tuesday, a fire brigade official said, after a similar incident on Saturday claimed 15 lives.
Mumbai is looking to turn itself into a global financial hub but large parts of the city struggle to cope with annual monsoon rains, as widespread construction and garbage-clogged drains and waterways make it increasingly vulnerable to chaos.
More than 300mm of rain fell over 24 hours in some areas of Mumbai, flooding streets and railway tracks, forcing the suspension of some suburban trains, which millions of commuters ride to work each day.
About 1,000 people stranded in low-lying areas of the city were rescued after a swollen river began to overflow, municipal authorities said.
As weather officials forecast intermittent heavy showers and isolated extremely heavy rainfall, authorities called a holiday for government offices and educational institutions.
"Rain is expected to remain intense even today," city authorities said on Twitter. "We request you to stay indoors unless there's an emergency."
The Indian navy has deployed various teams for rescue and relief works in Mumbai.
Financial markets opened normally on Tuesday, though trading volumes were expected to be lower than usual. Many firms asked their employees to work from home.
The main runway at Mumbai airport, India's second-biggest, was closed from midnight after a SpiceJet flight overshot the runway while landing, an airport spokeswoman said.
The secondary runway is operational, but 55 flights were diverted and another 52 were cancelled due to bad weather, she said.
In 2005, floods had killed more than 500 people in Mumbai, the majority in shantytown slums home to more than half the city's population.