Authorities in Fiji are assessing the damage after the most powerful storm left at least five people dead.
Cyclone Winston brought winds of over 320km/h (200mph), torrential rain and waves of up to 12m (40ft).
It destroyed hundreds of homes and cut electricity lines. There are reports of entire villages flattened.
The government has imposed a nationwide curfew and 30-day state of national disaster giving extra powers to police to arrest people without a warrant.
The Category-five storm - the highest level - moved westward since making landfall at 18:30 local time (06:30 GMT) on Saturday in the north of Fiji's main island, Viti Levu.
It changed direction at the last minute, sparing the capital Suva the full force of its winds.
The government had opened about 750 evacuation centres.
More than 1,000 people were sheltering in one on the second largest island of Vanua Levu, north-east of Viti Levu, the Fiji Broadcasting Company said.
Among the victims was an elderly man who died on the smaller Koro Island when a roof fell on him.
Some villages reported that all homes had been destroyed, Jone Tuiipelehaki of the UN Development Programme tweeted after the storm hit.
He said 50 homes in Navaga village on Koro Island had been reported ruined.
"The images that we're starting to see roll in are terrifying," Alice Clements, from the UN children's organisation Unicef in Suva told Reuters news agency. She said she could see a car on a building roof and a small plane stuck in debris.
In the north coast of Fiji's main island where the cyclone made landfall, a man told Reuters the damage was so extensive that "it looks like a different country".
George Dregaso, of Fiji's National Disaster Management Office, told the Associated Press that about 80% of the nation's 900,000 people were without regular electricity supplies.
Schools have been ordered to shut for a week even though the main airport has been reopened to receive humanitarian supplies.
Prime Minister Frank Bainimarama described the storm on Saturday as "an ordeal of the most grievous kind".
"When we are able we will provide timelines for the return of water and power," he said.
New Zealand and Australia have promised to send aircraft to assess the damage in remote areas.
Red Cross Pacific office head Ahmad Sami told AFP news agency humanitarian needs were likely to be "very high".
The cyclone has now moved out to sea, although strong winds and heavy rains remained likely, Fiji's Meteorological Office said.