Federal Home Minister Rajnath Singh called the situation "alarming" and said that the state capital, Chennai, had turned into an island.
Since November, he said another 54 people have died in the heavy rains in the neighbouring state of Andhra Pradesh.
Floods worsened in Chennai after water was released into the Adyar River from a overflowing reservoir. Highways leading into the city have been damaged and the waterlogged international airport will remain closed until Sunday, Singh said.
At least 2 000 disaster relief and military personnel were deployed, Singh said, adding that ongoing poor weather was making relief work difficult.
Following an aerial survey of the devastation, Prime Minister Narendra Modi said the Tamil Nadu government would receive 10 billion rupees ($150m) in federal assistance.
Hundreds were stranded at home, food was running short and ATM machines were not working, NDTV news channel reported.
The Metereological Department predicted more heavy rainfall in the region until Saturday.
The coastal city and its hinterland have received two spells of very heavy rain in the October-December wet season, largely due to low-pressure formations over the nearby Bay of Bengal.
Over 40% of telephone connections were down and the electricity had to be disconnected in several waterlogged areas for the safety of residents, Singh said.
The floods in Chennai were caused by unregulated urbanisation and climate change-induced freak weather, Sunita Narain, director general of New Delhi-based Centre for Science and Environment said.
"In Chennai, each of its lakes has a natural flood discharge channel which drains the spillover. But we have built over many of these water bodies, blocking the smooth flow of water ... We only see land for buildings, not for water," Narain said.