Since the start of the season devastating floods across South Asia have killed more than 950 people and affected close to 40 million across northern India, southern Nepal and northern Bangladesh. At least six people, including two toddlers, were among the victims in and around India's financial capital. Elephants were pressed into service, wading through swirling waters to rescue people, and aid workers have built rafts from bamboo and banana leaves.
Floods have washed away tens of thousands of houses and led to landslides in Nepal, Bangladesh, and India.
In the state of Bihar in India, 17.1 million people have been impacted, with 514 killed.
Bihar, Odisha, Uttar Pradesh and West Bengal were among the major states affected by floods in 2017.
Poor areas of Nepal have been particularly hard-hit; more than 210,000 homes have been damaged or destroyed by floods or landslides, and 143 people have died.
The IFRC - working with the Bangladesh Red Crescent Society and the Nepal Red Cross - has launched appeals to support nearly 200,000 vulnerable people with immediate relief and long-term help with water and sanitation, health and shelter.
Weather officials predict heavy seasonal monsoon rains in the city and its surroundings over the next 24 hours, following more than 100 mm (4 inches) of rain in Mumbai on Tuesday, and urged people to stay indoors.
The rain reduced visibility, forcing airport authorities to divert a few flights. The majority of deaths occurred in shanty town slums, home to more than half of Mumbai's population.
The education minister has asked all schools and colleges in the city to remain shut on Wednesday.
On Tuesday several firms made arrangements to provide food and rest areas for employees stuck in offices, while officials of temples and religious bodies offered help to those stranded on streets.