PARIS — Notre-Dame cathedral, the symbol of the beauty and history of Paris, was scarred by an extensive fire on Monday evening that caused its delicate spire to collapse, bruised the Parisian skies with smoke and further disheartened a city already back on its heels after weeks of violent protests.

The spectacle of flames leaping from the cathedral’s wooden roof — its spire glowing red then turning into a virtual cinder — stunned thousands of onlookers who gathered along the banks of the Seine and packed into the plaza of the nearby Hôtel de Ville, gasping and covering their mouths in horror and wiping away tears.