Shock, prayers around the world for Notre Dame Cathedral

UN chief "horrified" as Notre Dame burns

United Nations Secretary-General Antonio Guterres expressed his "horror" at the images of the fire, shown live around the world.

"Horrified by the pictures coming from Paris with the fire engulfing Notre Dame Cathedral -- a unique example of world heritage that has stood tall since the 14th century. My thoughts are with the people and government of France," the UN chief said on Twitter.

Audrey Azoulay, head of UNESCO -- the Paris-based UN agency for education, science and culture -- also expressed her "deep emotion" on social media.

Azoulay wrote that UNESCO is closely monitoring the situation and is standing by the French people's side to "safeguard and restore this invaluable heritage."

The UN elevated Notre Dame, widely considered to be one of the finest examples of French gothic architecture, to world heritage status in 1991.

Obamas express sympathy with French people

Former US President Barack Obama posted an old photo of himself, his wife Michelle and their two daughters lighting candles in the cathedral.

"Notre Dame is one of the world's great treasures, and we're thinking of the people of France in your time of grief," Obama said on Twitter. Michelle Obama, who was in Paris on Monday on a book tour, said "my heart aches with the people of France."

"The majesty of Notre Dame — the history, artistry, and spirituality — took our breath away, lifting us to a higher understanding of who we are and who we can be," she tweeted. She predicted the cathedral would rise again.