The cathedral, one of France's finest, was painted several times by impressionist artist Claude Monet in the 19th century.
Television images on BFM channel showed a dark plume of smoke rising from the spire and people in the streets below looking up in horror.
The scene was reminiscent of the devastating fire at Notre-Dame cathedral in Paris in 2019, which also started during renovation works.
The Rouen cathedral spire had been surrounded by scaffolding and a white cover for several weeks.
The local prefecture, which exercises state authority in the region, said the cathedral had been evacuated and emergency services were on the scene. A security cordon was in place around the building.
The prefecture said there were no reports of casualties and the extent of the damage was unclear for now.
(Reporting by Inti Landauro and Estelle Shirbon; Editing by Toby Chopra, Alexandra Hudson)

A viewer inspects a series of paintings of Rouen Cathedral by French artist Claude Monet at a press preview of Monet and Architecture at the National Gallery in London, England. (Photo: EPA-EFE / Will Oliver)
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