* Public prosecutor says no sign of arson
* Billionaires, companies pledge donations for rebuild
* 3D diagram of Notre-Dame: https://tmsnrt.rs/2DgH76t
* Map and timeline: https://tmsnrt.rs/2DgHcXP
* (Adds quote)
By Richard Lough and Elizabeth Pineau
PARIS, April 16 (Reuters) - The fire that tore through Notre-Dame cathedral was probably caused by accident, French prosecutors said on Tuesday after firefighters doused the last flames in the ruins overnight and the nation grieved for the destruction of one of its symbols.
More than 400 firemen were needed to tame the inferno that consumed the roof and collapsed the spire of the eight-centuries-old cathedral. They worked through the night to extinguish the fire some 14 hours after it began.
Paris public prosecutor Remy Heitz said there was no obvious indication the fire was arson. Fifty people were working on what would be a long and complex investigation. One firefighter was injured but no one else was hurt in the blaze which began after the building was closed to the public for the evening.
From the outside, the imposing bell towers and outer walls, with their vast flying buttresses, still stood firm, but the insides and the upper structure were eviscerated by the blaze.
Firefighters examined the gothic facade and could be seen walking atop the belfries as police kept the area in lockdown.
Investigators will not be able to enter the cathedral's blackened nave until experts are satisfied its stone walls withstood the heat and the building is structurally sound.
The fire swiftly ripped through the cathedral's timbered roof supports, where workmen had been carrying out extensive renovations to the spire's wooden frame.
The Paris prosecutor has opened an investigation into "involuntary destruction by fire". Police on Tuesday began questioning the workers involved in the restoration, the prosecutor's office said.
Hundreds of stunned onlookers had lined the banks of the Seine river late into the night as the fire raged, reciting prayers and singing liturgical music in harmony as they stood in vigil.
"Yesterday we thought the whole cathedral would collapse. Yet this morning she is still standing, valiant, despite everything. It is a sign of hope," said Sister Marie Aimee, a nun who had hurried to a nearby church to pray as the fire spread.
It was at Notre-Dame that Napoleon was made emperor in 1804, Pope Pius X beatified Joan of Arc in 1909 and former presidents Charles de Gaulle and Francois Mitterrand were mourned.
Messages of condolence flooded in from around the world.
Pope Francis, the leader of the Catholic Church, was praying for those affected, the Vatican said, adding: "Notre-Dame will always remain - and we have seen this in these hours - a place where believers and non-believers can come together in the most dramatic moments of French history."
Britain's Queen Elizabeth expressed deep sadness while her son and heir Prince Charles said he was "utterly heartbroken".
VOW TO REBUILD
President Emmanuel Macron promised to rebuild Notre-Dame, considered among the finest examples of European Gothic architecture, visited by more than 13 million people a year.
Notre-Dame is owned by the state. It has been at the centre of a years-long row between the nation and the Paris archdiocese over who should finance badly needed restoration work to collapsed balustrades, crumbling gargoyles and cracked facades.
It was too early to estimate the cost of the damage, said the heritage charity Fondation du Patrimoine, but it is likely to run into the hundreds of millions of dollars.
The rival billionaire owners of France's two biggest luxury fashion empires, Francois-Henri Pinault of Kering and Bernard Arnault of LVMH, pledged 100 million euros and 200 million euros to the restoration respectively. Oil company Total pledged 100 million. The city of Paris said it would provide 50 million.
Paolo Violini, a restoration specialist for Vatican museums, said the pace at which the fire spread through the cathedral had been stunning.
"We are used to thinking about them as eternal simply because they have been there for centuries, or a thousand years, but the reality is they are very fragile," Violini said.
The company carrying out the renovation works when the blaze broke out said it would cooperate fully with the investigation.
"All I can tell you is that at the moment the fire began none of my employees were on the site. We respected all procedures," Julien Le Bras, a representative of family firm Le Bras Freres.
Officials breathed a sigh of relief that many relics and artworks had been saved. At one point, firefighters, policemen and municipal workers formed a human chain to remove the treasures, including a centuries-old crown of thorns made from reeds and gold, and the tunic believed to have been worn by Saint Louis, a 13th century king of France.
"Notre-Dame was our sister, it is so sad, we are all mourning," said Parisian Olivier Lebib. "I have lived with her for 40 years. Thank God that the stone structure has withstood the fire." (Reporting by Sudip Kar-Gupta, Inti Landauro, Richard Lough, Sarah White, Emmanuel Jarry and Luke Baker in Paris; Additional reporting by Philip Pullela Writing by Richard Lough; Editing by Leigh Thomas, Raissa Kasolowsky and Peter Graff)
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