The heaviest rainfall in over a century has caused massive flooding Tamil Nadu, driving thousands from their homes, shutting auto factories and paralysing the airport in the state capital Chennai.
Prime Minister Narendra Modi has blamed climate change for the torrential rains, injecting urgency into the debate at global climate talks in Paris and highlighting the vulnerability of tropical nations like India to extreme weather.
Chennai, India's fourth most populous city, is a major auto manufacturing and IT outsourcing hub. Ford Motor, Daimler, Hyundai and Nissan told workers to stay at home on Wednesday, while U.S. listed outsourcing firm Cognizant shut its 11 local offices.
Indian airlines suspended flights into Chennai's flooded international airport, causing wider disruption to air travel.
"The biggest challenge is to find a way to clear the inundated airport and main roads," said Anurag Gupta at the National Disaster Management Authority (NDMA) in New Delhi.
Passengers stranded at the airport said they did not know when they would be able to fly, or where to stay if they could not.
"All of us here are getting agitated because none of the hotels nearby are vacant. Where do we go?" traveller Vinit Jain told Reuters Television.
No deaths were reported and the extent of damage would only become clear when the floodwaters receded, another NDMA official said. The federal home ministry said 18 people had suffered flood-related injuries.
Weather experts say the seasonal northeast monsoon was responsible for the flooding in the city of six million, which like many other parts of India lacks proper drainage systems.
Jatin Singh, founder of private weather forecaster Skymet, said the northeast monsoon was typically more intense in years like this when El Nino - or a warming of the waters in the eastern Pacific Ocean - prevailed.
At least twice as much rain fell in the last 24 hours as the average for the whole month of December, Skymet said, adding that the downpour would continue for another 24 hours.
Tamil Nadu is a major rice and sugar cane producing region, and a senior member of a local farmers association said floods had washed out up to four agricultural districts.
Modi has ordered rescue teams and paramilitary forces to launch an extensive relief and rescue operation in Chennai.
He had blamed climate change for the heavy rains that hit the southern state last month, tweeting before attending the UN climate summit in Paris this week: "We are feeling the impact of fast-paced climate change."
Hundreds of divers and army rescue teams entered inundated homes, taking the injured to hospital. Authorities said more than a million people were affected by the flooding, with some residents bemoaning the slow response of the relief teams.
"The police want to help but there are no boats. We are trying not to panic," said Ramana Goda, who took refuge at a police station after fleeing his home with his family overnight.
Television footage showed people wading through chest-deep water and the infirm being carried shoulder-high on makeshift stretchers. Others were stranded on rooftops.
Social media networks carried many appeals for help, while others offered assistance. Siddarth, a popular Tamil film actor who goes by one name, was coordinating a relief effort on Twitter.
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