A fire at the cathedral in the French city of Nantes is believed to have been started deliberately, prosecutors say.
Three fires were started at the site and an investigation into suspected arson is under way, Prosecutor Pierre Sennes said.
The blaze destroyed stained glass windows and the grand organ at the Saint-Pierre-et-Saint-Paul cathedral, which dates from the 15th Century.
It comes a year after the devastating fire at Notre-Dame cathedral in Paris.
But the local fire chief said the fire in Nantes had been contained and was "not a Notre-Dame scenario".
"The damage is concentrated on the organ, which appears to be completely destroyed. The platform it is situated on is very unstable and risks collapsing," Laurent Ferlay told reporters.
The cathedral roof had not been touched by the blaze, he said.
President Emmanuel Macron has reacted, tweeting: "After Notre-Dame, the St Peter and St. Paul Cathedral is in flames. Support to the firefighters who are taking all the risks to save the Gothic jewel."
The French fire service tweeted footage of the blaze.
The fire began in the early morning, with massive flames visible from outside the building. More than 100 firefighters brought it under control after several hours.
Mr Sennes said the national police would be involved in the investigation and a fire expert was travelling to Nantes
"When we arrive at a place where a fire has taken place, when you see three separate fire outbreaks, it's a question of common sense, you open an investigation," he said.
Newsagent Jean-Yves Burban said he heard a bang at about 07:30 local time (05:30 GMT) and saw flames when he went out to see what was happening.
"I am shook up because I've been here eight years and I see the cathedral every morning and evening. It's our cathedral and I've got tears in my eyes," he told Reuters.
This is not the first fire at the cathedral. It was damaged by Allied bombing in 1944, during World War Two, and then in 1972 its roof was largely destroyed.
It was rebuilt 13 years later with a concrete structure replacing the wooden roof.
In 2015 a huge fire destroyed part of the 19th Century basilica of Saint-Donatien in Nantes.