Cyclone Yasa, with winds at its centre gusting as high as 350km/h, made landfall on the island of Vanua Levu.
The category five storm was over the province of Bua, but a hurricane warning was in force for nearly the whole country.
A state of disaster had already been declared, and a curfew was in effect for the entire country.
The National Disaster Management Office said at least 600,000 people were in Yasa's path.
Prime Minister Frank Bainimarama said Cyclone Yasa could easily surpass the devastation of 2016's Cyclone Winston, which killed 44 people, destroyed 40,000 homes and significantly impacted 350,000 people in the country.
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