At least two people confirmed dead and thousands displaced in Andhra Pradesh state.
Tropical Cyclone Amphan has developed in the Bay of Bengal and is heading north towards eastern Indian states of Odisha and West Bengal before bearing down on western Bangladesh.
This enormous and slow-moving cyclone has a cloud band which is about 2,200km (1367 miles) wide and a well-defined 15km (9 miles) round eye. It continues to strengthen.
On Monday afternoon, the storm was located about 500km (310 miles) to the southeast of the port city of Visakhapatnam, in Andhra Pradesh state, and is packing sustained winds of 260km (161 miles) per hour. The gusts are approaching 315km/h (195 miles/h).
Amphan is equivalent to a Category 5 major Atlantic hurricane on the Saffir-Simpson scale and is expected to strengthen further. Winds of 270km/h (168 miles/h) gusting 325km/h (202 miles/h) are possible before it gradually begins to weaken off the coast of Odisha.
The entire system is moving very slowly northwards across the warm waters of the Bay of Bengal at about 11km/h (7 miles/h). Current projections have the centre of the storm making landfall to the southeast of Kolkata, close to the border with West Bengal and Bangladesh at about 12:00 GMT on Wednesday.
It should have weakened significantly by then with winds falling to approximately 150km/h (93 miles/h) and gusts approaching 185km/h (115 miles/h, equivalent to a Category 1 Atlantic hurricane), but the usual risks will still apply.
The cyclone is still producing wave heights of about 15 metres (49 feet) and a storm surge of 9 metres (30 feet). This catastrophic storm will still produce damaging winds and combined with heavy and steady rainfall mass flooding is likely as the storm moves inland.
Total rainfall of 200-300mm in 48-72 hours is expected and some spots in northern Bangladesh could see as much as 450mm rain.
A huge population is likely to be affected by the storm. Kolkata alone has a population of 14 million, while most of Bangladesh’s 170 million people live within the low-lying flood plains of the very vulnerable Ganges Delta.