PARIS (AP) — A catastrophic fire engulfed the upper reaches of Paris' soaring Notre Dame Cathedral as it was undergoing renovations Monday, threatening one of the greatest architectural treasures of the Western world as tourists and Parisians looked on aghast from the streets below.

The blaze collapsed the cathedral's spire and spread to one of its landmark rectangular towers, but Paris fire chief Jean-Claude Gallet said the church's structure had been saved after firefighters managed to stop the fire spreading to the northern belfry. The 12th-century cathedral is home to incalculable works of art and is one of the world's most famous tourist attractions, immortalized by Victor Hugo's 1831 novel "The Hunchback of Notre Dame."

There was shock and disbelief in San Diego as parishioners attended mass at St. Joseph Cathedral. At Our Lady of Sacred Heart Church in City Heights, heartbreak expressed among parishioners. 

The fire came less than a week before Easter amid Holy Week commemorations. As the cathedral burned, Parisians gathered to pray and sing hymns outside the church of Saint Julien Les Pauvres across the river from Notre Dame while the flames lit the sky behind them.

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Father Patrick Mulcahy of St. Joseph Cathedral in downtown San Diego said the experience he had inside Notre Dame was special. 

"It was about seven years ago. I was on a trip with a group of people. It is powerful to walk into the cathedral. It is more than just a building. It is in the heart of Paris. It is a visible sign of the presence of God," he said. 

Built in the 12th and 13th centuries, Notre Dame is the most famous of the Gothic cathedrals of the Middle Ages as well as one of the most beloved structures in the world. Situated on the Ile de la Cite, an island in the Seine river, its architecture is famous for, among other things, its many gargoyles and its iconic flying buttresses.

Among the most celebrated artworks inside are its three stained-glass rose windows, placed high up on the west, north and south faces of the cathedral. Its priceless treasures also include a Catholic relic, the crown of thorns, which is only occasionally displayed, including on Fridays during Lent.

"The magnificent stained glass windows are some of the most important in the world ever created and inspired artists for generations. There are also paintings and sculptures inside the cathedral. It is a huge treasure trove," said Anita Feldman, Deputy Director of Curatorial Affairs at the San Diego Museum of Art. 

The exact cause of the blaze was not known, but French media quoted the Paris fire brigade as saying the fire is "potentially linked" to a 6 million-euro ($6.8 million) renovation project on the church's spire and its 250 tons of lead. The Paris prosecutors' office ruled out arson and possible terror-related motives, and said it was treating it as an accident.