As rescuers raced to reach the survivors, images from Mayotte showed scenes of devastation, with homes reduced to piles of rubble.
After summoning key government officials, Macron announced he would visit the archipelago "in the coming days", declaring the most destructive cyclone to hit Mayotte in 90 years a national catastrophe.
"In the face of this tragedy, which has shaken each and every one of us, I will declare a national mourning," he posted on X after the evening crisis meeting.
Cyclone Chido is the latest in a string of storms worldwide fuelled by climate change, according to experts.
The disaster poses a major challenge for a government still only operating in a caretaker capacity, days after Macron appointed the sixth prime minister of his presidency.
It left health services in tatters, with the main hospital extremely damaged and health centres knocked out of operation, Health Minister Genevieve Darrieussecq told France 2 television.
Offering its "deepest condolences", the United States said it was prepared to provide humanitarian assistance to those affected by the storm.
- Climate change super-charge -
After visiting Mayotte on Monday, Interior Minister Bruno Retailleau declared the territory "completely devastated".
"The slums, the shantytowns, there's nothing left of them," he said.
Cyclone Chido caused major damage to Mayotte's airport and cut off electricity, water and communication links when it barrelled down on Saturday.
Trees were uprooted and power lines knocked down.
Supplying fresh drinking water, a problem on Mayotte even in normal times, is now a major priority.
"We're starting to run out of water. In the south, there's been no running water for five days," said Antoy Abdallah, a resident of Tsoundzou in the territory's capital Mamoudzou.
"We're completely cut off from the world," the 34-year-old lamented.
Half of the territory's running water would be restored within 48 hours, according to the interior ministry.
There was also widespread damage to telecommunications, with mobile phone networks, internet access and fixed-line services almost entirely knocked out, telecom providers in Mayotte reported.
The "exceptional" cyclone was super-charged by particularly warm Indian Ocean waters, meteorologist Francois Gourand of the Meteo France weather service told AFP.
In Brazil, host of the next UN climate change conference, the foreign ministry said Monday that the cyclone highlighted the need for increased global efforts at adapting to the consequences of climate change.
- 'Shanty towns flattened' -
It will take days before the full death toll becomes clear, according to Retailleau.
Prefect Francois-Xavier Bieuville, the top Paris-appointed official on the territory, told broadcaster Mayotte la Premiere: "I think there will definitely be several hundred, perhaps we will come close to a thousand or even several thousand."
With roads closed, officials fear that many could still be trapped under rubble in inaccessible areas.
Most of Mayotte's population is Muslim and religious tradition dictates bodies must be buried rapidly, meaning some may never be counted.
Mayotte is France's poorest region, with an estimated third of the population living in shantytowns whose flimsy sheet metal-roofed homes offered scant protection against the storm.
"All the shantytowns are flattened, which suggests a considerable number of victims," a source close to the authorities told AFP, asking not to be named.
And assessing the toll is further complicated by irregular immigration to Mayotte, especially from the Comoros islands to the north.
Mayotte officially has 320,000 inhabitants, "but it is estimated that there are 100,000 to 200,000 more people, taking into account illegal immigration," the source added.
The source said few unregistered residents would have gone to the accommodation centres before the cyclone, "probably for fear of being checked".
- 'Apocalyptic scenes' -
Chido was packing winds of at least 226 kilometres (140 miles) per hour when it slammed into Mayotte, which lies to the east of Mozambique.
One resident, Ibrahim, told AFP of "apocalyptic scenes" as he made his way through the main island, having to clear blocked roads himself.
The nearby French island of La Reunion was serving as a hub for the rescue operations, with hundreds of French security personnel being deployed.
As authorities assessed the scale of the disaster, a first aid plane reached Mayotte on Sunday.
It carried three tonnes of medical supplies, blood for transfusions and 17 medical staff, according to authorities in La Reunion.
Ousseni Balahachi, a former nurse, said some people did not dare venture out to seek assistance, "fearing it would be a trap" designed to remove them from Mayotte.
Many had stayed put "until the last minute" when it proved too late to escape the cyclone, she added.
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