A fierce cyclone has ripped along India's east coast, killing at least five people and forcing about half a million into overcrowded shelters as the storm threatened to cut a swathe of devastation through farmland and fishing hamlets.

Gusts over land could hit speeds of almost 300km/h in one of the largest cyclones the country has ever seen.

Around 12 million people are expected to be affected, officials said.

Muslims and Hindus gathered at mosques and temples in Odisha state, praying Phailin would not be as devastating as a similar storm that killed 10,000 people 14 years ago.

Heavy rain pounded coastal villages in neighbouring Andhra Pradesh.

Wind speeds reached at least 220km/h, and were expected to cause a 3.4m surge in sea levels when the storm hit the coast, the India Meteorological Department said.

"The storm has high damage potential, considering windspeed," Lakshman Singh Rathore, head of the weather department, said.

Families trekked through the rain to shelters, television images showed, as gusts of wind snapped branches from trees. Tourists fled Puri, a popular beach resort.

Officials broadcast cyclone warnings through loudspeakers, radio and television.

"The wind speed is picking up," said Odisha's Special Relief Commissioner, Pradeep Kumar Mohapatra. "Some people were earlier reluctant to move. They are willing now."

In nearby Andhra Pradesh, heavy rain and strong winds pummelled a coastal highway and left lush green fields sodden with water.

London-based Tropical Storm Risk said the storm was already in that category, and classed it as a Category 5 storm - the strongest.

The US Navy's weather service said wind at sea was gusting at 314km/h.

Authorities warned of extensive damage to crops, village dwellings and old buildings, as well as disruption of power, water and rail services. Shelters were being stocked with rations, and leave for government employees was cancelled.