France24 reports that the peak of the cathedral has been undergoing a €6 million renovation project and the fire could be linked to the renovation work.
Built over a century starting in 1163, Notre-Dame is considered to be among the finest examples of French Gothic cathedral architecture.
The fire came less than a week before Easter amid Holy Week commemorations.
"Like all our countrymen, I'm sad tonight to see this part of us burn", French President Emmanuel Macron said in a statement.
"From our hotel room we watched the big tall spire go straight down into the nave of the church", Debbie Furr said. One firefighter was seriously injured - the only reported casualty. He pledged to rebuild the church and said he would seek global help to do so.
UK Prime Minister Theresa May has said her thoughts are "with the people of France tonight and with the emergency services who are fighting the bad blaze at Notre-Dame cathedral".
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Paris Archbishop Michel Aupetit invited priests across France to ring church bells in a call for prayers for the beloved Paris cathedral.
The disaster appeared to resonate with the president, who called Notre Dame "one of the great treasures of the world" and declared it to be "beyond countries, that's beyond anything".
Historian Camille Pascal spoke to French broadcaster BFMTV, saying the fire was destroying "invaluable heritage".
Alain Faubert, the auxiliary bishop of the Archdiocese of Montreal, said whenever he goes to Paris he makes a point to stop by the cathedral.
The pastor of Montreal's Notre-Dame Basilica, Miguel Castellanos, said "words are not enough" to describe what he feels about the fire.
"Everything is burning. The framing, which dates from the 19th century on one side and the 13th on the other, there will be nothing left", Andre Finot, a spokesman for Notre Dame, told Agence France-Presse.
He added: "We can be only horrified by what we see".