Indian weather service says cyclone Fani is expected to make landfall on Friday afternoon in Odisha state and barrel northeastwards on a pathway close to the homes of more than 100 million people.
Nearly 800,000 people in eastern India have been evacuated from the expected path of a major cyclone packing winds up to 200 kilometres per hour and torrential rains, officials said on Thursday.
The Indian weather service said Extremely Severe Cyclonic Storm Fani was expected to make landfall on Friday afternoon in Odisha state and barrel northeastwards on a pathway close to the homes of more than 100 million people.
A state relief department official told AFP that 780,000 people were moved to safer places overnight from at least 13 districts of Odisha, home to some 46 million people, which will bear the brunt of the weather system.
"We are expecting more than a million people to move out of the danger zone in next 12 hours," Bishnupada Sethi, Odisha Special Relief Commissioner, told AFP.
Some 3,000 shelters in schools and government buildings have been set up to accommodate more than a million people.
More than 100,000 dry food packets are ready to be dropped if needed, reports said.
On Thursday the storm, which reports said was the biggest to hit eastern India in nearly two decades, was brewing in the Bay of Bengal some 250 kilometres offshore and moving slowly but ominously westwards.
The cyclone was expected to pack sustained wind speeds of 180-190 kph, bringing gusts of up to 200 kph, equivalent in strength to a Category 3 to 4 hurricane.
It was expected to make landfall near the Hindu holy town of Puri, a major tourist hotspot attracting millions of Indian and foreign visitors every year.
More than 100 trains have been cancelled in past 48 hours, according to a statement by Indian Railways. Three special trains were running from Puri to evacuate pilgrims and tourists.
Authorities have asked tourists to leave coastal areas and avoid unnecessary travel.
Special buses have been deployed in Puri and other towns.
Dozens of officials were making announcements on hand-held megaphones across the coastal belt asking residents to leave their homes for safety.
The Indian Navy has also been put on alert.
The Airports Authority of India issued an advisory to all the coastal airports to take adequate precautions.
"Our men are urging people to move to safer places and government has set up sufficient shelters to take in those evacuating their homes," said Sethi.
"Heavy rains are expected in all the coastal districts amid fears of flash floods. We are all geared up for the challenge."
Forecasters have predicted "heavy to very heavy" rain on Friday in some places and "extremely heavy" rain the next day, with some areas inundated by up to 20 centimetres (8 inches) of water.
Tides could also surge up to 1.5 metres (five feet).
India's weather office has warned that the high speed winds can uproot trees, flatten crops and damage homes, power and communication infrastructure.
The neighbouring coastal states of Andhra Pradesh, home to 50 million people, and Tamil Nadu, population some 70 million, have also been put on high alert.
Fani will be the fourth such storm to hit India's east coast in three decades. Major weather systems regularly affect eastern and southeastern India between April and December.
In 2017, Cyclone Ockhi left nearly 250 people dead and more than 600 missing in Tamil Nadu and Kerala states.
The worst cyclone on record in Odisha in 1999, killed almost 10,000 people and caused an estimated $4.5 billion worth of devastation.
Odisha had to evacuate some 300,000 people last October when its coastal districts were battered by Cyclone Titli, with winds up to 150 kph and heavy rains. At least two people died.