The storm passed roughly 100 km (62 miles) south of Mayotte around 0900 GMT on Sunday, according to forecaster Météo-France.
“Extremely heavy rains are starting to fall,” François-Xavier Bieuville, prefect of Mayotte, told French news channel BFMTV.
They caused the first floods “and relatively significant landslides” across the territory, he added.
The prefect said the island was likely to remain on red alert until Monday night, as heavy rain was expected to continue even after the storm passed.
At least 14,500 people took shelter in emergency shelters set up to protect them from the storm, BFMTV reported.
On Sunday afternoon, the storm was moving away from Mayotte, Météo-France reported. The system is expected to slowly intensify over the next 24 hours to tropical cyclone status as it approaches the coast of Mozambique.
The current forecast does not predict landfall in Mozambique, but the Nampula region is expected to experience “very degraded conditions”, the forecaster said.
Mozambique is also recovering from Cyclone Chido which killed 120 people in that country.
