Crowds of stunned Parisians and tourists — some crying, others offering prayers — watched on in horror in central Paris on Monday evening as flames engulfed the Notre-Dame Cathedral.

Gasps and cries of "Oh my god" erupted at 7:50pm (1750 GMT) when the top portion of the church's spire came crashing down into an inferno that has spread to the entire roof.

More gasps came a few seconds later when the rest of the spire collapsed, caught on the cameras of thousands of mobile phones.

"Paris is disfigured. The city will never be like it was before," said Philippe, a communications worker in his mid-30s, who had biked over after being alerted of the fire by a friend.

"I'm a Parisian, my father was a Parisian, my grandfather as well — this was something we brought our sons to see," he said. "I won't be showing this to my son."

"It's a tragedy," he added. "If you pray, now is the time to pray."

Police were attempting to clear pedestrians away from the two islands in the river Seine, including the Ile de la Cite which houses the soaring Gothic church, one of Europe's best known landmarks.

But throngs of onlookers kept trying to approach, snarling traffic as they massed on the stone bridges leading to the islands.

Another woman passed by, tears streaming from behind her glasses, too overwhelmed to speak to reporters.

"It's finished, we'll never be able to see it again," said Jerome Fautrey, a 37-year-old who had come to watch.

"Now we need to know how this happened — with everything that's going on in the world, why Notre-Dame? Maybe it's a message from on high," he said.

'History up in smoke'

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, the cathedral’s 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 cathedral was immortalized in Victor Hugo’s “The Hunchback of Notre-Dame”, published in 1831, and has long been a subject of fascination in popular culture as well as the traditional art world.

“Everything is burning, nothing will remain from the frame,” Notre-Dame spokesperson Andre Finot told French media.

French historian Camille Pascal told BFM broadcast channel the blaze marked “the destruction of invaluable heritage”.

“It’s been 800 years that the Cathedral watches over Paris,” Pascal said. “Happy and unfortunate events for centuries have been marked by the bells of Notre-Dame.”

He added: “We can be only horrified by what we see.”

"It's incredible, our history is going up in smoke," said Benoit, 42, who arrived on the scene by bike.

Sam Ogden, 50, had arrived from London on Monday with her husband, their two teenaged sons, and her mother. They had come to Paris specifically to see Notre-Dame, part of a world tour over years to see historic sites.

"This is really sad — the saddest thing I've ever stood and watched in my life," Ogden said.

She said the fire looked tiny at the beginning, "then within an hour it all came down."

Her mother, Mary Huxtable, 73, said: "This [Notre-Dame] was on my bucket list to see. Now I'll never go inside."

Header image: Flames rise from Notre-Dame cathedral as it burns in Paris on Monday, April 15. — AP