Pope Francis has said he's "pained" by Turkey's decision to convert Istanbul's Hagia Sophia back into a mosque.
Speaking at a service in the Vatican, the Roman Catholic leader added that his "thoughts go to Istanbul".
Hagia Sophia was built as a Christian cathedral nearly 1,500 years ago and turned into a mosque after the Ottoman conquest of 1453.
The Unesco World Heritage Site became a museum in 1934 under Turkish Republic founding father Ataturk.
But earlier this week a Turkish court annulled the site's museum status, saying its use as anything other than a mosque was "not possible legally".
Pope Francis confined himself to a few words on the issue: "My thoughts go to Istanbul. I think of Santa Sophia and I am very pained."
President Recep Tayyip Erdogan said the first Muslim prayers would be held in Hagia Sophia on July 24.
Shortly after the announcement, the first call to prayer was recited at the site and broadcast on all of Turkey's main news channels. Hagia Sophia's social media channels have also been taken down.
Islamists in Turkey have long called for it to become a mosque again but secular opposition members opposed the move.
Defending the decision, President Erdogan stressed that the country had exercised its sovereign right, and he added that the building would remain open to all Muslims, non-Muslims and foreign visitors.
'Voices not heard'
The Pope is one of several religious and political leaders worldwide who have criticised the move.
The World Council of Churches has called on President Erdogan to reverse the decision. The Church in Russia, home to the world's largest Orthodox Christian community, immediately expressed regret that the Turkish court had not taken its concerns into account when ruling on Hagia Sophia.
It has also drawn condemnation from Greece, and Unesco said its World Heritage Committee would now review the monument's status.
One of Turkey's most famous authors, Orhan Pamuk, told the BBC that the decision would take away the "pride" some Turks had in being a secular Muslim nation.
"There are millions of secular Turks like me who are crying against this but their voices are not heard," said Mr Pamuk.
History of a global icon
- Hagia Sophia's complex history began in the year 537 when Byzantine emperor Justinian built the huge church overlooking the Golden Horn harbour
- With its huge dome, it was believed to be the world's largest church and building
- It was briefly a Catholic cathedral in the 13th Century when Crusaders captured the city
- In 1453, in a devastating blow to the Byzantines, Ottoman Sultan Mehmed II captured Istanbul (formerly known as Constantinople) and the victorious conqueror performed Friday prayers inside Hagia Sophia
- The Ottomans soon converted the building into a mosque, adding four minarets to the exterior and covering ornate Christian icons and gold mosaics with panels of Arabic religious calligraphy
- After centuries at the heart of the Muslim Ottoman empire, it was turned into a museum in 1934 in a drive to make Turkey more secular
- Today Hagia Sophia is Turkey's most popular tourist site, attracting more than 3.7 million visitors a year