Cyclone Chido began battering the French Indian Ocean territory of Mayotte on Saturday, after authorities issued the highest alert and ordered everyone, including rescue workers, to seek shelter.
"It's an unprecedented event, extremely violent," Governor Francois-Xavier Bieuville said.
As the cyclone neared the territory, the authorities banned all 320,000 residents from moving.
A resident on the main island of Grande Terre, Ibrahim Mcolo, described fallen electricity masts, roofs ripped off homes and trees uprooted as the first gusts struck.
"There is no more electricity," he told AFP from his home, where he had barricaded himself in.
"Even in our house, which is well protected, the water is getting in. I can feel it trembling."
Raising the highest alert "entails the strict confinement of the entire population, including rescue and security services and all officials mobilised for crisis management", the local government wrote on social media platform X.
The warning went into force at 7:00 am (0400 GMT), before the eye of the storm was due to pass over northern Mayotte.
"This is serious. Mayotte has never known a situation like it," local council leader Ben Issa Ousseni said on Friday.
230-kph winds possible
The authorities have turned more than 70 schools and gyms into shelters. They have given priority to the 100,000 residents assessed as living in the most vulnerable homes in France's poorest territory.
"We're really scared," said Fatima, a resident of the village of Majicavo-Koropa on the eastern coast.
The 57-year-old told AFP she had stocked up on bottles of water, food and candles.
The French state weather service said in a bulletin at 0600 GMT that the eye of the storm was about 30 kilometres (19 miles) from the east of the archipelago.
It said the gusts were faster than 180 kilometres per hour and could reach as much as 230 kph.
Road traffic was banned and the airport of Dzaoudzi was closed.
The regional health agency said: "Medical resources have been reinforced to take care of injured or sick people."
The mayor of the town of Ouangani, Youssouf Ambdi, said he feared "the worst".
"There is sure to be material damage. We are praying there are no victims."
The weather service forecast that conditions would improve from late on Saturday.
A cyclone alert was also in place and airports closed in the neighbouring Comoros archipelago, and residents reported flooding in Madagascar to the east.
