"I think of Hagia Sophia and I am very saddened," Pope Francis said towards the end of his midday sermon in Saint Peter's Square. 
A magnet for tourists worldwide, the Hagia Sophia was first constructed as a cathedral in the Christian Byzantine empire but was converted into a mosque after the Ottoman conquest of Constantinople in 1453.
Turkey’s president, Recep Tayyip Erdoğan, announced on Friday that Muslim prayers would begin on 24 July at the Unesco world heritage site.