The blaze is on the "top floor of the building and the roof", a fire service spokesperson said.
He said no one was staying in the five-star hotel and there were no casualties but a "large" part of the building was affected.
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Firefighters were called at 6:00am to the blaze at the hotel on the Place Vendome, in the heart of Paris' luxury quarter.
Paris police reported a "major fire" on their Twitter account and advised motorists to avoid the area.
The four-storey classically styled hotel, owned by Egyptian billionaire Mohamed Al Fayed, was due to reopen late this year after a three-year closure for top-to-bottom renovations.
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The hotel of choice of Charlie Chaplin, Coco Chanel and Ernest Hemingway, where a small bar is named after him, the Ritz is also infamous as the place where Princess Diana and Dodi Fayed dined before their fatal car crash in 1997.
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