Hours after a blaze engulfed the roof of the centuries-old Notre Dame Cathedral, distraught Parisians and stunned tourists lined the banks of the Seine River, gazing in disbelief. Christians sang liturgical songs such as Hail Mary and Ave Maria, as the inferno raged at the cathedral. Others knelt and prayed. Thousands of onlookers stopped at bridges over the Seine and along its embankments, held at a distance by a police cordon. Notre Dame sits on the Ile de la Cite, an island on the Seine River marking the very centre of Paris. The Paris prosecutor's office said it had launched an inquiry into the fire. Several police sources said that they were working on the assumption for now that it was accidental.