Panicked family members raised the alarm after the one-and-a-half-year-old boy went missing from home on Wednesday night, with a 50-member search team of disaster and local civic officials deployed to find the toddler.
The boy was later seen on CCTV falling into a drain overflowing with monsoon rain.
"Early enquiry reports have revealed that the cover of the drain was removed by certain individuals. CCTV footage is being checked & investigated to trace the (boy)," the local municipal body said in a tweet.
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Searchers were using heavy machinery to bring down walls along an overground section of the drainage line, the India Today website reported.
A civic official told reporters the boy was likely to have been washed away due to the heavy flow of water inside the drain.
Angry residents gathered at the site and threatened to block roads if the toddler was not found soon.
The incident comes as Mumbai reels under the summer monsoon when heavy rains cause havoc for its 20 million residents.
Citizens blame government apathy, accusing local authorities of failing to maintain roads and buildings properly.
Activists say contractors hired to repave and maintain roads do a bad job on purpose so the work will need to be done again the following year.
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