Worst hit were the northeastern Indian state of Assam, the two northern states of Bihar and Uttar Pradesh and neighboring Bangladesh, where incessant rains have caused dozens of swollen rivers to burst their banks and inundate the surrounding regions.
"I have not seen such flooding in the last 24 years. It's a sheet of water everywhere," Santosh Mishra, a resident of the Gonda district in Uttar Pradesh, said after authorities urged residents of 65 villages to evacuate.
"There are no signs of houses, temples or trees," Mishra told the local Sahara Samay television channel.
Some 14 million people in India and 5 million in Bangladesh were displaced or marooned by the flooding, according to government figures, with at least 120 people killed in recent days in India and 46 more in Bangladesh.
"The situation is grim," said Bhumidhar Barman, a minister in the Assam state government.
In New Delhi, India's Meteorological Department said unusual monsoon patterns this year led to heavier than usual rains in these regions, while central India had only light rains. "We've been getting constant rainfall in these areas for nearly 20 days," said B. P. Yadav, a spokesman for the department.
The monsoon season in South Asia runs from June to September. More than 1,000 people died last year, with most deaths blamed on drowning, landslides, house collapses or electrocution.
In Assam some 100,000 displaced people were staying in government relief camps while hundreds of thousand more sought shelter on higher ground, setting up makeshift dwellings. Millions more were cut off from the rest of the country.
"Strong currents have washed away 1,640 yards of the highway and a wooden bridge on the only alternate road got swept by the flood waters," said Dibakar Misra, a government official in the worst hit Dhemaji region in Assam.
He said railway services had been suspended after a long stretch of tracks was damaged and that people had to be rescued in boats as waters were 30 feet deep in some places.
Atul Deka, a farmer, described how he watched helplessly as swirling waters washed away the bamboo footbridge connecting his village to the road across the stream.
"We couldn't do anything as it happened in a flash. Now, we have to depend on the few row boats we have until the floods recede and we build the bridge all over again," said Deka, from the Satdola village, on the outskirts of Gauhati, the capital of Assam.
Meanwhile, medical teams were trying to visit different regions by boat to make sure there were no outbreaks of waterborne diseases such as cholera.
"There is no outbreak of any epidemic and things are under control," said Himanta Biswa Sharma, Assam's health minister.
A similar situation existed in Bihar, where 120 relief centers had been set up, said Manoj Srivastava, a state disaster management official. Others were camped out on highways, he said.
In Uttar Pradesh the army was called in to help evacuate people from 500 villages under water, said Diwakar Tripathi, a senior government official, adding that crops worth millions of rupees (hundreds of thousands of dollars) had been destroyed.
On Wednesday, 28 people died when the overcrowded boat evacuating them from their village sank in a swollen river.
In Bangladesh, a low-lying delta nation of 145 million people, the River Jamuna breached its banks, inundating much of Sirajganj, a small town 64 miles northwest of the capital, Dhaka.
Schools and some government offices have been closed as streets were under waist-high water.