Half-a-million people have been forced to flee their homes as India faced its biggest cyclone since the turn of the century.
Aid agencies are gearing up to help those worst affected by the storm, though the full extent of the damage - including a death toll - will not be known until daybreak.
The storm, which made landfall early Saturday night near the town of Golpalpur in Orissa state, has already caused large-scale power and communications outages and shut down road and rail links, officials said. It also sent people clamouring for safety, seeking shelter in refuges.
Roads were all but empty on Saturday as high waves pounded the coastline of Orissa state. Seawater pushed inland, swamping villages where many people survive as subsistence farmers in mud and thatch huts.
As the cyclone swept across the Bay of Bengal towards the Indian coast on Saturday, satellite images showed its spinning tails covering an area larger than France.
A cyclone of similar strength 14 years ago killed 10,000 people.
Children's charity World Vision said its teams on the ground were already seeing signs of the storm.
Dharmendra Naik, manager of its programmes in Jagathsinghpur district, said: "Our staff along the coast have been seeing rain falling continuously and winds pick up. People have been trying to stock up on essentials and that has caused the price of many items to be driven higher."
Kunal Shah, the head of the charity's emergency response in India, said: "In a storm of this magnitude there is the potential for widespread damage to crops and livestock in the low-lying coastal areas and houses completely wiped away. So while we are praying this storm loses intensity, we're also preparing."
Several of the charity's programmes are in the direct path of the cyclone. Staff have been alerting communities and providing megaphones, life jackets, torch lights and ropes to the Community Task Force, which consists of men, women and youngsters from within the community who are trained in disaster preparedness including search and rescue, basic first aid and protecting livestock.
Save the Children said it was on the ground in Odisha where the cyclone is expected to hit, stockpiling emergency supplies including food, health and hygiene kits and tarpaulin sheets.
Plan International is also in contact with partners in India, ready to intervene to help families affected.
Emergency response manager Murali Kunduru said: "Children are particularly vulnerable during emergencies and their specific needs must be considered by the humanitarian community. Child protection must be the prime concern of the government, relief agencies and parents."
Families directly in the path of the storm have been evacuated to safe areas by government agencies. District heads have been asked to stockpile food, medicine and other aid in case needed.
Mr Kunduru said: "Plan is working in co-ordination with the government and other humanitarian actors to meet the food, shelter, water, protection and education needs of those affected."
The Indian Meteorological Department warned that Phailin was a "very severe cyclonic storm" that was expected to hit with maximum sustained winds of 130-135 miles per hour.
However, the US Navy's Joint Typhoon Warning Centre in Hawaii forecast maximum sustained winds of 167 miles per hour with gusts up to 196 miles per hour.
University of Miami hurricane researcher Brian McNoldy said that Phailin is near the size of 2005's Hurricane Katrina, which killed 1,200 people and caused devastating flooding in New Orleans, and it also has the wind power of 1992's Hurricane Andrew, which had 165mph winds at landfall in Miami.
If the storm continues on its current path without weakening, it is expected to cause large-scale power and communications outages and shut down road and rail links, officials said. There would also be extensive damage to crops.
Satellite images of the storm showed its spinning tails reaching nearly 1,000 miles from the east coast of India to the west coast of Burma, an area roughly the size of France.
Devendra Tak, of Save the Children, in Puri, Odisha, said: "Our plane took four attempts to land in Bhubaneswar on Saturday morning because conditions were so bad - and as soon as we disembarked they shut the airport.
"As we drove towards the coast we passed uprooted trees, damaged houses and towns like ghost towns as most people had been evacuated to cyclone shelters. When we got to the coast the waves were extremely high - the sea was like a cauldron of boiling water.
"It's getting windier and the rain is increasing hour on hour. The cyclone is about to make landfall any time now and we will begin to know the extent of its damage overnight."
Oxfam said that in villages where it works, people have been evacuated to safe shelters.
Its contingency stock of water sanitation materials are on standby and ready for deployment, and the Oxfam India humanitarian hub has staff on standby for assessment and initial immediate response in the affected areas.
Speaking from Bhubaneswar, Ghasiram Panda, of ActionAid, said: "ActionAid has authorised the purchase of food, bottled water and other essential items such as soap and sanitary products for evacuees.
"Scarcities of food and a lack of clean drinking water are already being reported by our teams on the ground and with the storm not expected to be over until midday Sunday, protection of children, pregnant women, the disabled and the elderly is a priority."
In Bhubaneswar, the Orissa state capital, government workers and volunteers were putting together hundreds of thousands of food packages for relief camps.
The state's top official, chief minister Naveen Patnaik, said: "I request everyone to not panic. Please assist the government. Everyone from the village to the state headquarters have been put on alert," he told reporters