President Emmanuel Macron has vowed to rebuild Notre-Dame "even more beautifully" within five years, as all of France's cathedrals prepared to ring their bells on Wednesday to mark 48 hours since the colossal fire began.

The blaze on Monday gutted the great Paris landmark, destroying the roof, causing the steeple to collapse and leaving France reeling with shock.

Macron announced the speedy timescale for restoration -- a process some experts had said would take decades -- in an address to the nation where he hailed how the disaster had shown the capacity of France to mobilise and unite.

Pledges worth around 700 million euros ($790 million) have already been made from French billionaires and businesses to restore the Gothic masterpiece.

An unknown number of artefacts and paintings have been lost and the main organ, which had close to 8,000 pipes, has also suffered damage.

But the cathedral's walls, bell towers and the most famous circular stained-glass windows at France's most visited tourist attraction remain intact.

- Rebuilding in time for Olympics -

In a further sign of the monument's resilience, the copper rooster that topped its spire was found Tuesday in the rubble from the partly collapsed roof, "battered but apparently restorable" according to a spokesperson for the culture ministry.

Macron's announcement of a five-year restoration timeframe indicates he wants the reconstruction to be completed by the time Paris hosts the Olympic Games in 2024.

"We will rebuild the cathedral even more beautifully and I want it to be finished within five years," he said in the speech from the presidential palace. "We can do it."

Macron said that the dramatic fire had brought out the best in a country riven with divisions and since November shaken by sometimes violent protests against his rule.

It had shown that "our history never stops and that we will always have trials to overcome," he added.

The bells of all cathedrals in France will sound at 6:50 pm (1650 GMT) on Wednesday, 48 hours after the fire started.

Investigators trying to determine the cause of the blaze will also continue questioning workers who were renovating the steeple on Monday before the fire broke out.

The police, who suspect the operation to replace the steeple's lead covering may have triggered the disaster, have already spoken to around 30 people from five different construction companies.

Public prosecutor Remy Heitz has said the investigation promises to be "long and complex".

- Structural 'weaknesses' -

Images from inside the cathedral showed its immense walls standing proud, with statues still in place and a gleaming golden cross above the altar.

However the floor was covered in rubble and scorched beams from the fallen roof and water while parts of the vaulting at the top of the cathedral had collapsed.

Junior interior minister Laurent Nunez told reporters that work to secure the structure would continue into Thursday.

He said the building had been saved within a critical time window of 15-30 minutes by a team of 400 firefighters who worked flat out through the night.

Though "some weaknesses" in the 850-year-old structure had been identified, overall it is "holding up OK", he added.

A public appeal for funds has secured nearly a billion euros in pledges from some of France's wealthiest families and companies.

French billionaire Bernard Arnault and his LVMH luxury conglomerate, rival high-end designer goods group Kering, Total oil company and cosmetics giant L'Oreal each pledged 100 million euros or more.

Support came from outside France as well, with Apple chief Tim Cook announcing the tech giant would give an unspecified amount.

But experts had warned a full restoration will take many years.

"I'd say decades," said Eric Fischer, head of the foundation in charge of restoring the 1,000-year-old Strasbourg cathedral.

Germany, Italy and Russia are among the countries to have offered expert help.

- Treasures saved -

Thousands of Parisians and tourists watched in horror Monday as flames engulfed a building that has symbolised Paris for nearly a millennium.

Many more came Tuesday to the banks of the river Seine to gaze at where the roof and steeple once stood and a prayer vigil at the nearby Saint-Michel square drew hundreds of worshippers.

A 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 13 million tourists each year.

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 IX.

Rescuers formed a human chain at the site of the disaster to evacuate as many artefacts as possible, which were stocked temporarily at the Paris town hall.