A fire at a 15th-century cathedral in the French city of Nantes is being treated as a criminal act, prosecutors say. 

The fire, which started on Saturday morning at the cathedral of St Peter and St Paul, took dozens of firefighters several hours to tackle. 

A number of the cathedral's stained glass windows were blown out by the strength of the fire, while the building's grand organ was also destroyed. 

"We've observed this morning that the fire started in three spots," said Nantes prosecutor Pierre Sennes. "This observation leads us to open an investigation for arson, an investigation to regional judicial police.

"The next phase will be working into the investigation consultations – we will take a lot of statements. We will also verify the technical installations, the fire prevention installations, and again, we are waiting for experts who could give us interesting elements." 

Even though the fire has now been tackled, it will hit the entire city hard according to mayor Johanna Rolland.

"What overcomes us is the emotion, what overcomes us is the sadness, because this place is not only important and emblematic for Nantes' Catholics – it's important for all Nantes residents. It's part of our history, there's a very emotional link to it."

This is not the first fire to hit the cathedral, which was started in 1434 and not completed until 1891. It was damaged by bombing during World War II and then again in 1972. On that occasion its timber roof was largely destroyed, and the restoration work was so in-depth that the cathedral stayed closed for 13 years. 

Rolland was quick to ease fears that this fire's effects were not as devastating. "I can confirm to the locals that ask us if we are in the same situation as in 1972 – since this question keeps coming up – no, we cannot reasonably say that right now, we are in the same situation as 1972."

France's president Emmanuel Macron linked the fire to last year's blaze at Paris's main cathedral. "After Notre-Dame, the St Peter and St. Paul Cathedral is in flames," Macron tweeted. "Support to the firefighters who are taking all the risks to save the Gothic jewel."