Xi sends condolences to French president over Notre Dame fire

One firefighter was injured but no one else was hurt, with the fire starting at around 6.30 pm after the building was closed to the public for the evening.

Firefighters examined the facade, with its spectacular 10-metre filigreed stained-glass rose window still intact. They 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 walls withstood the heat and the building is structurally sound.

The fire swiftly ripped through the cathedral's oak roof supports, where workmen had been carrying out extensive renovations to the spire's timber-framed supports.

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 river Seine late into the night, reciting prayers and singing as they stood in vigil while the fire raged.