Firemen were battling a fire that broke out early on Saturday morning inside the cathedral in the western French city of Nantes, the local fire service said on Twitter.
TV images showed smoke coming out of the 15th century structure. The blaze comes just over a year after a major fire at the Notre Dame cathedral in Paris, which destroyed its roof and main spire.
Cecile Renaud, who works in a bakery facing the cathedral and alerted the fire services early on Saturday, told BFM TV she had seen huge flames inside the building.
“It was a huge shock. It’s extremely sad.”
It is not the first time the cathedral has caught fire. It was partly destroyed during World War Two in 1944 after Allied bombings. In 1972 a fire completely ravaged its roof. It was finally rebuilt 13 years later.