Cyclone Fani has slammed into India's eastern coastline, according to the country's Meteorological Department.
Heavy rain has been reported in the eastern state of Orissa, also called Odisha, which is directly in the path of the storm.
The tourist town of Puri and neighbouring areas are experiencing winds with a speed of 175 km/h (108mph), which may go up to 200 km/h.
Officials say there are no casualties.
However flooding has been reported from several areas, and reports are coming in of trees falling and roofs of buildings being destroyed.
More than a million people have been evacuated from Orissa and moved to shelters while several neighbouring states have also been put on high alert.
The cyclone is expected to hit 15 districts in Orissa, apart from Puri, which is home to the 858-year-old Jagannath temple.
Numerous flights and train services in and out of the state have been cancelled, while schools and government offices are shut. Operations at three ports on India's eastern coast have been shut down.
The state and federal governments have swung into action - naval warships and helicopters have been moved to the state and are on standby with medical teams and relief material. The country's National Disaster Response Force (NDRF) has also sent several teams there.
In 2017, Cyclone Ockhi killed more than 200 people and displaced hundreds. In October last year, officials in Orissa evacuated hundreds of thousands of people when another cyclone struck.
The state's most deadly cyclone on record took place in 1999, killing almost 10,000 people.
Which areas will be affected?
India's National Disaster Management Authority has warned people along the east coast, especially fishermen, not to go out to sea because the conditions are "phenomenal".
The agency said the "total destruction of thatched houses" was possible, as well as "extensive damage" to other structures.
The cyclone is expected to move towards Chittagong in Bangladesh in a weaker form on Saturday.
The cyclone coincides with high tides in Bangladesh which may exacerbate potential flooding issues there.
The port city of Cox's Bazar, where hundreds of thousands of Rohingya refugees live in camps with minimal shelter, is also on alert. But the cyclone is currently not expected to hit the camps unless it changes path, which is not thought likely.
In February the International Federation of Red Cross and Red Crescent Societies (IFRC) began distributing tarpaulins ahead of the region's "cyclone season" - but warned that if a deadly storm rolls in, shelters made of battered bamboo and shredded plastic would offer little protection.
How has India prepared?
The navy, the coast guard and the National Disaster Response Force have all been prepared for deployment. It has also stationed two ships with divers and doctors at the southern port cities of Vishakapatnam and Chennai (formerly Madras), an official told local media.
Local media report that about 81 trains travelling to and from coastal cities have been cancelled.
India's electoral commission has relaxed its rules about what the government can do during election periods so that the authorities can carry out relief work.
The country is in the middle of a multi-phased election which started last month.
Under normal circumstances the incumbent government has certain powers suspended, so that it can't announce new schemes or take new decisions during the voting period.
Although the election will continue until the end of May, Orissa has already voted.
https://www.bbc.com/news/world-asia-india-48121606
