Authorities expected more rains on Tuesday as two days of downpour flooded roads and disrupted rail and air traffic in the financial capital Mumbai.
Wall collapses in Mumbai and nearby towns, caused by the worst monsoon rains in a single day in 14 years, killed 30 people on Tuesday and disrupted rail and air traffic, prompting officials to shut schools and offices.
Financial markets were open in the city of 18 million touted as a potential rival to the Chinese city of Shanghai, but hampered by poor infrastructure like many other Indian cities.
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 on shanties built on a hill slope in Malad, a western suburb of Mumbai, killing 21 people, a fire brigade official said.
Three people died when a school wall collapsed in the city of Kalyan, 42 kilometres (26 miles) north of Mumbai.
In the nearby western city of Pune, six people were killed in a wall collapse on Tuesday, a fire brigade official said, after a similar incident on Saturday killed 15.
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.
As much as 375 millimetres (14.8 inches) of rain fell over 24 hours in some areas of Mumbai, the highest in 14 years, flooding streets and railway tracks, forcing the suspension of some suburban train services, which millions of commuters ride to work each day.
About 1,000 people stranded in low-lying areas of the city were rescued by naval personnel using rubber boats 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.
Indian TV showed images of flooded homes and people walking through waist-deep water, stoking criticism of city authorities.
"Every year, the first spell of rainfall throws normal life out of gear in Mumbai. An inquiry is needed into why this happens despite claims of preparations," said Ajit Pawar, a state opposition leader.
Devendra Fadnavis, chief minister of Maharashtra state, which includes Mumbai, said the city's infrastructure cannot handle excessive rainfall in a short period of time, but new pumping stations would be operational soon.
Many firms asked 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.
"Our team is trying their best to bring the main runway back in operation and this may take up to 48 hours," airport authorities said later on Twitter.