Firefighters have brought under control a blaze that began inside the Nantes Cathedral on Saturday morning, officials said.
The Cathedral of Saint Pierre and Saint Paul in Nantes is a gothic-style structure that dates back to the 15th century.
Some 100 firefighters were deployed to put out the fire, whose flames could be seen through the church's front windows as smoke poured out of them. No one was injured.
Curious residents and tourists crowded around the structure as emergency workers cordoned off much of the city's historical center on the Loire River.
Authorities said the blaze was limited to the church pipe organ. Local fire department chief Laurent Ferlay said the organ was "completely destroyed" and that the platform on which it sits was found on the verge of collapse.
French prosecutors said arson was suspected, noting the blaze had erupted in "three separate places."
"An arson investigation has been opened, there are no conclusions to be drawn now because we have a lot of investigations to make which could introduce new elements," Nantes prosecutor Pierre Sennes told AFP.
According to the Nantes diocese, the cathedral had already survived a major fire in 1972 that destroyed its roof. The church was reopened in 1985 after extensive restoration.
Saturday's blaze comes a little more than a year after a large fire at the iconic Notre Dame Cathedral in the capital, Paris, destroyed the roof of the 12th-century Gothic structure, including its 93-meter-high (305-feet-high) spire.
Although the Nantes cathedral fire was not as devastating as that of Notre Dame, the French government is taking it seriously.
French Prime Minister Jean Castex and French Interior Minister Gerald Darmanin are said to travel to Nantes Saturday afternoon to meet with local authorities on the scene.
jcg/mm (AP, AFP, dpa)