Churches around Greece were in "mourning" on Friday (July 24) as Muslim prayers were to be held at Hagia Sophia in Istanbul for the first time in decades.

Church bells around the country pealed 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" of 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.

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 are expected to hold protests in Athens and Thessaloniki later on Friday.