Churches around Greece were in "mourning" Friday as Muslim prayers were to be held at Hagia Sophia in Istanbul for the first time in decades, with protests by religious groups due later.
Church bells around the country will peal at midday with their flags at half-mast to protest what the head of the Church of Greece, Archbishop Ieronymos, has called an "unholy act of defiling" the former Byzantine Empire cathedral.
"(Today) is a day of mourning for all of... Christianity," Ieronymos said.
The archbishop said he would hold a special service at the Athens Metropolis in the evening and chant the Akathist Hymn in honour of the Virgin Mary.
According to Greek tradition, the same service was held in Hagia Sophia on the eve of the Byzantine's Empire's fall to the Ottomans in 1453.
Hagia Sophia is "a symbol of our faith and a universal monument of culture," Ieronymos said.
Religious and nationalist groups will hold protests in Athens and Thessaloniki later Friday.
One of the architectural wonders of the world, the UNESCO World Heritage site in Istanbul was the main cathedral of the Byzantine Empire but was converted into a mosque after the Ottoman conquest of Constantinople in 1453.
A top Turkish court revoked the sixth-century monument's status as a museum on July 10.
Turkish President Recep Tayyip Erdogan then ordered the building to reopen for Muslim worship, deeply angering the Christian community and further straining relations with NATO ally Greece.
Erdogan went ahead with the plan despite appeals from the United States and Russia and condemnation by France and Pope Francis.
Greece's culture ministry has called it "a provocation to the civilised world".
Hagia Sophia in 2007 was on a shortlist of global architectural wonders selected by nearly 100 million Internet and telephone voters.