Cyclone Winston smashed into Fiji late Saturday, bringing with it wind gusts as high as 325km/h, to the main island Viti Levulate.
The country had spent much of Saturday in lockdown, with transport cancelled and a nationwide curfew and state of emergency declared by the government.
There were initial reports of severe damage across the country, including some of houses being flattened and heavy swells washing away sea walls.
Winston, the strongest cyclone ever recorded in the South Pacific, had previously hit the country's eastern Lau group of islands. Authorities said contact with the scattered group had been lost and it was unclear what the extent of the damage was.
Meteorological Service director Ravind Kumar said the scenario was not good.
"It is a category five system, it is very compact, and it has got very destructive hurricane force winds.
"Over the island of Vanuabalavu we recorded winds gusting to 145 knots (268km/h), so you can imagine the destruction that it may leave behind," he said.
Late on Saturday night, there were reports of widespread blackouts and communications outages across the country, particularly in the north and east.
Curfew and State of Emergency
Fiji had spent many days bracing itself for impact of the cyclone.
The country went into a curfew at 6 o'clock on Saturday evening, with only essential services permitted to travel. The it remains in place.
The government also declared a 30-day nationwide state of emergency in anticipation of widespread destruction, by what was predicted to be one of the most destructive cyclones to ever hit the country of close to one million people.
"As a nation, we are facing an ordeal of the most grievous kind," said Prime Minister Frank Bainimarama said in a statement urging people to heed authorities' warnings.
"Some people in urban areas of the country in particular do not appear to have heeded the warnings about the seriousness of the threat we all face," he said.
More than 700 evacuation centres had been activated throughout Fiji, and disaster authorities urged people in low-lying and exposed areas to evacuate before the cyclone set in.
Public transport and flights were cancelled, and the Fiji Roads Authority urged people to avoid driving on the roads unless absolutely necessary.
Many markets and businesses were ordered to close by the government, and people in the northern towns of Labasa and Savusavu were called on to evacuate.
A resident in Savusavu on Vanua Levu, Sharon Wild, said the cyclone was starting to strike the island with incredible ferocity.
Ms Wild said it was the strongest she had ever experienced, and her family had taken to sheltering in the bathroom.
"It's crazy outside, you can't imagine," she said in a phone interview, her voice trembling.
"The coconut trees are swaying right over, I can't even see the island outside. I have no idea how my house is - if I still have a roof on my house. We have the sliding glass doors that are just shaking and they look like they're going to explode, so actually we're all sitting in the bathroom - we're all in the bath."
Unicef Fiji spokesperson Alice Clements said authorities and agencies had been preparing for Winston's arrival for days, and had stockpiles of relief supplies.
"The government's been taking it very seriously and it's been doing a great job of reaching out to communities, making sure everyone's aware of where the evacuation centres are, providing constant updates through the weather service and through television, radio and online announcements," Ms Clements said.
As the tropical cyclone begun its assault on Fiji, Mr Bainimarama said: "Let us all pray for our nation, ourselves and each other."