French investigators interviewed construction workers involved in renovation work at the Notre-Dame Cathedral on Tuesday as they sought to identify the source of a devastating blaze at the monument that has sent shockwaves through France and the world.

Donations and offers of help began to pour in as day broke over the City of Light, revealing the extent of the damage from Monday night's inferno which took around 15 hours to extinguish.

"All night long I saw men going past with tears in their eyes. I described it this way: It was total chaos, but we can't let it knock us down," said Philippe Marsset, the vicar general of Notre-Dame.

Most of the roof has been destroyed, the steeple has collapsed and an unknown number of artifacts and paintings have been lost, but the walls, bell towers and the most famous circular stained-glass windows remain in tact.

Read: Spire collapses at Paris’s Notre-Dame cathedral after ferocious fire

Around 400 firefighters battled through the night to control the flames and the last remnants of the inferno were extinguished early on Tuesday, 15 hours after it broke out.

Ongoing renovation work is widely suspected to have caused the fire after the blaze broke out in an area below scaffolding.

Investigators interviewed witnesses overnight and began speaking to the employees of five different construction companies which were working on the monument, said public prosecutor Remy Heitz.

"Nothing indicates this was a voluntary act," Heitz told reporters, adding that 50 investigators had been assigned to the case.

A public appeal for funds drew immediate support from French billionaires and other private donors as well foreign countries including Germany, Italy and Russia which offered to lend their expertise.

French billionaire Bernard Arnault announced on Tuesday that he and the LVMH luxury conglomerate he controls would give 200 million euros ($226 million), after luxury rival Kering offered 100 million euros.

In pictures: 'Paris is disfigured': Tears and shock as Notre-Dame burns

Specialised craftsmen and rare materials are also expected to be needed to restore the monument, seen as symbol of Western civilisation and an emblem of France which has survived revolutions and wars.

A French wood company in Murlin, central France, offered to put its loggers to work immediately for the oak beams that will be needed to rebuild the vaulted roof of the UNESCO-listed World Heritage site.

"The work will surely take years, decades even, but it will require thousands of cubic metres of wood," Sylvain Charlois of the Charlois group told France Inter radio.

The privately run French Heritage Foundation launched a call for donations on its website while several pages were set up on the Leetchi fundraising portal.

"We have everything to be able to rebuild it in exactly the same way," Culture Minister Franck Riester told France Inter, adding that the government was looking at ways to encourage donations.

Full losses unknown

The inferno destroyed two-thirds of the roof of the 850-year-old landmark, whose spectacular Gothic spire collapsed as orange flames and clouds of grey smoke billowed into the evening sky.

One firefighter suffered injuries during the blaze, which at one point threatened to bring down one of the two monumental towers on the western facade of the cathedral that is visited by around 13 million tourists each year.

Culture minister Riester warned that the structure remained unstable and that no one had yet been able to fully inspect the extent of the fire and water damage inside.

"We'll have to wait for a while to do a full inventory," he said.

The Holy Crown of Thorns, believed to have been worn by Jesus at his crucifixion, was saved by firefighters, as was a sacred tunic worn by 13th-century French king Louis.

Rescuers formed a human chain at the site of the disaster to bring out as many artifacts as possible which have been stocked at the Paris town hall.