A man has been arrested after a fire broke out at the gothic cathedral of Nantes in western France on Saturday, said the city's prosecutor.
The fire, started in three different parts of the cathedral, destroyed stained glass windows and the grand organ.
Prosecutors, who confirmed the arrest of the man, gave no more detail about the suspect's motivations.
Catholic officials mourned the loss of priceless artefacts and paintings as well as the 17th century organ – a star attraction of the cathedral.
Regional fire chief Laurent Ferlay however said the damage was not comparable to last year's devastating blaze at Notre-Dame cathedral in Paris.
Passers-by saw flames behind the rosette of the Cathedral of St Peter and St Paul and alerted emergency services a little before 8am local time.
Roughly 100 firefighters rushed to the scene and managed to save the structure, built between the 15th and 19th centuries, Mr Ferlay said.
Nantes prosecutor Pierre Sennes said the fire had started in three different places, so a judicial investigation for arson was immediately opened.
"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," Mr Sennes told AFP.
The prosecutor said investigators had found no sign of a break-in, but noted that one fire has started near the organ, while the other two were at the other end of the cathedral.
Catholic official Father François Renaud, who oversees the cathedral, surveyed the damage with firemen and told AFP the organ console had "completely disappeared", describing it as "an unimaginable loss".
"The console of the choir organ has gone up in smoke along with the adjoining wooden choir stalls. Original stained glass windows behind the great organ have all shattered," he said.
While the blaze was still raging, President Emmanuel Macron tweeted support for "our firefighters who are taking all kinds of risks to save this gothic jewel".
