Thousands in the tea-rich state of Assam took shelter in railway stations and government buildings as their homes filled with waist-deep water.
With many rivers running above danger levels, roads and bridges were washed away, disrupting food and medicine supplies to people displaced from their homes.
"Many people are facing a severe food shortage," said Mahesh Deka, a villager in Morigaon district, one of the worst affected areas of Assam.
India's monsoon season, which runs from June to September, causes flooding every year, killing hundreds of people.
The western state of Gujarat and central Madhya Pradesh were hit by heavy monsoon flooding this month which killed more than 130, but the waters have now ebbed there.
Officials said the mighty river Brahmaputra in Assam was flowing more than a metre (3 ft) above the danger mark.
The Brahmaputra, which rises in the Himalayas in Tibet, flows through Assam before entering Bangladesh and emptying into the Bay of Bengal.
"All rivers are overflowing, submerging human habitats and farmlands, causing misery to thousands of residents," a government statement said.
The local weather office recorded more than 72 mm of rainfall and warned of heavy rains in the region in the next 36 hours.
"The administration has been geared up to meet any eventuality," said Samsher Singh, a senior government official in charge of relief and rescue operations.
Flood waters have entered Kaziranga National Park, a world heritage site known for its single horn rhinos, forcing animals to migrate to nearby hills and highways.
Menacing floods, caused by intermittent rains and overflowing rivers, are an annual phenomenon in the state. Last year more than 200 people died in flooding and over two million were left homeless.
- Reuters - Thomson Reuters Foundation
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