A catastrophic fire at Paris's Notre-Dame cathedral has left a nation mourning the devastation of its cultural and historic "epicentre" and sent shockwaves around the world.

Hundreds of firefighters tackled the historic blaze through the night, battling to stop it wreaking complete destruction of the treasured facade after flames torched the roof, sending its spire crashing to the ground before crowds of horrified Parisians.

This is what we know so far about the violent blaze:

How did it start?

The Paris prosecutor's office said it was treating the fire as an accident, ruling out arson and possible terror-related motives, at least for now.

The fire began around 6.50 pm (5.50pm UK time) on Monday evening.

"I was not far away, I saw the smoke. At first I thought it was the Hotel-Dieu (hospital) but then I realised it was the cathedral. When I arrived, ash was beginning to fall," said Olivier De Chalus, head volunteer guide of the cathedral.