* Nearly 400,000 people flee to storm shelters
* Storm verges on becoming a "supercyclone"; could affect 12 mln people
* Operations at key eastern port halted; major gas field seen spared
Rain and wind lashed India's east coast and nearly 400,000 people fled to storm shelters after authorities issued a red alert and warned of major damage when one of the largest cyclones the country has ever seen hits land later on Saturday.
Filling most of the Bay of Bengal, Cyclone Phailin was about 300 km (187 miles) offshore on Saturday morning, satellite images showed, and was expected to reach land by nightfall.
The storm verged on becoming a "super cyclone" and was expected to affect 12 million people, 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.
Phailin was packing winds of at least 220 kph (137 mph) and was expected to cause a 3.4-m (11-foot) surge in sea levels when it hit the coast, the India Meteorological Department said in a statement.
"The storm has high damage potential, considering windspeed," Lakshman Singh Rathore, head of the weather department, said on Friday.
Families trekked through the rain to shelters, television images showed, as gusts of wind snapped branches from trees. Tourists left 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.
"We are ready to evacuate," said wiry-haired Jagdesh Dasari, 35, chief of the fishing village of Mogadhalupadu, which has 2,500 residents, as the rain poured down.
"If the waves come higher, the whole place will vanish."
London-based Tropical Storm Risk said the storm was already in that category, and classed it as a Category 5 storm - the strongest. The U.S. Navy's weather service said wind at sea was gusting at 314 kph.
Some forecasters likened its size and intensity to hurricane Katrina, which tore through the U.S. Gulf coast and New Orleans in 2005.
It also evoked memories of an Indian storm in 1999, when winds reaching speeds of 300 kph battered Odisha for 30 hours.
This time, however, the Odisha government said it was better prepared. Half a million people are expected to shelter in schools and other strong buildings when the storm hits, officials said. At least 60,000 people left their homes in neighbouring Andhra Pradesh on Friday.
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.
A police official said a rescue effort was launched for 18 fishermen stranded four nautical miles at sea from Paradip, a major port in Odisha, after their trawler ran out of fuel.
Paradip halted cargo operations on Friday. All vessels were ordered to leave the port, which handles coal, crude oil and iron ore. An oil tanker holding about 2 million barrels of oil, worth $220 million, was also moved, an oil company source said.
But the storm was not expected to hit India's largest gas field, the D6 natural gas block in the Cauvery Basin further down the east coast, field operator Reliance Industries said.
Katrina hit the U.S. Gulf coast on Aug. 29, 2005, killing about 1,800 people, including many in New Orleans where levees failed to hold back storm surges.
It was one of the six biggest hurricanes - also known as cyclones and typhoons - ever recorded and caused damage of around $75 billion.