Greece hit out on Friday at Turkey's decision to convert Hagia Sophia back into a mosque, saying it was a demonstration of "weakness", as churches around the country mourned the controversial move.
"What is happening in (Istanbul) this day is not a show of force, but proof of weakness," Greek prime minister Kyriakos Mitsotakis said in a statement.
Nevertheless, the move would not "diminish the radiance of a global heritage monument", he said.
"Especially to us Orthodox Christians, Hagia Sophia today is in our hearts more than ever. It is where our heart beats," Mr Mitsotakis said.
On Friday, Turkish President Recep Tayyip Erdogan joined thousands in the first Muslim prayers in Hagia Sophia since the Istanbul landmark was reconverted from a museum into a mosque.
The Unesco World Heritage site was originally the Byzantine Empire's main cathedral before its first conversion into a mosque after the Ottoman conquest of Constantinople in 1453.
In 1934, modern Turkey's founder Mustafa Kemal Ataturk ordered it be turned into a museum, but a Turkish court said earlier this month it had been registered in property deeds as a mosque, paving the way for the Muslim worship to be reintroduced on Friday.
At midday, churches around Greece rang their bells and flew their flags at half-mast in protest against what the head of the Church of Greece, Archbishop Ieronymos, called an "unholy act of defiling" the former cathedral.
"(Today) is a day of mourning for all of... Christianity," Ieronymos said.
However, Constantinos Bogdanos, a deputy from the ruling New Democracy party, told AFP: "It is not a day of mourning, it is a day of revelation of Turkey's growing aggressiveness."
Religious and nationalist groups also staged protests in Athens and Thessaloniki.
Later on Friday, the archbishop held a special service at the Athens Metropolis, during which he chanted 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 Empire capital's fall to the Ottomans.
Hagia Sophia is "a symbol of our faith and a universal monument of culture," Ieronymos said.
On July 10, Turkey's highest administrative court, the Council of State, cancelled the 1934 decision to turn Hagia Sophia into a museum, saying it was registered as a mosque.
Erdogan then ordered the sixth-century monument be reopened for Muslim worship, deeply angering the Christian community and further straining relations with NATO ally Greece.
Erdogan pursued the plan despite appeals from the United States and Russia and condemnation by Pope Francis.
