It is indeed a very sad day for not only the French, but also humanity as well, as the Notre-Dame Cathedral is one of the world's greatest landmarks.

Here's what we know so far:

What happened?

At aroud 6.50pm French time (around 12.50am in Malaysia) on Monday, a massive fire broke out at the Notre-Dame Cathedral, which was built around 1260 AD.

According to a report by news agency Reuters, the flames quickly spread and burst rapidly through the roof of the cathedral, destroying the building's wooden interior and iconic stained-glass windows.

The spire and roof of the cathedral later collapsed:

The moment #NotreDame’s spire fell pic.twitter.com/XUcr6Iob0b

— Patrick Galey (@patrickgaley) April 15, 2019

French President Emmanuel Macron told ABC News that around 500 firefighters were mobilised to help fight the fire.

Areas surrounding the building were evacuated and the public were urged to avoid the area.

Despite the warning, thousands of people gathered in the streets, some openly weeping and some sang hymns, watching the flames engulf the building. Several churches around Paris also rang their bells in response to the blaze.

Four hours later, Paris Fire Brigade Commander General Jean-Claude Gallet told BBC News that they managed to save the main structure of the cathedral from total destruction. Its two iconic towers, according to Gallet, have been saved as well.

How did the fire start?

While an investigation is under way, CNN reported that it was not immediately known what caused the fire, although the ongoing restoration and renovation work could be the culprit.

Gallet told the news site that the fire department was first alerted to a fire which started in the attic of the cathedral.

The 850-year-old Gothic building went under renovation when cracks appeared in the stone and parts of it were crumbling, the BBC reported, sparking fears that the structure could become unstable.

According to ABC News, the restoration work costs €120mil (RM558mil) and that a fire alarm test was conducted just last week.

Was there casualty?

Thankfully, no casualty was reported. However, The Guardian reported that one firefighter was severely injured.

The Associated Press reported that firefighters also managed to salvage priceless arts and artifacts from the cathedral, including a centuries-old crown of thorns made from reeds and gold and the tunic worn by Saint Louis, a 13th century king of France.

The 16 statues of the apostles were removed from the cathedral last week for restoration work, according to the report.

What happens next?

French President Emmanuel Macron, while visibly shaken, has vowed to rebuild the Notre-Dame Cathedral by utilising "the best talents."

"The worst was avoided even if the battle has not been completely won," he was quoted by ABC News as saying. "We will rebuild Notre-Dame together".

The Independent UK reported that the Paris prosecutor's office has started an inquiry into the fire, which has been classified as "an accident" for now.