Pope Francis has said he is "deeply pained" by Turkey's decision to convert Hagia Sophia into a mosque.
The Byzantine landmark in Istanbul was built 1,500 years ago as a Christian cathedral and became a mosque after the Ottoman conquest in 1453. It has been a museum since 1934 and is a UNESCO World Heritage Site.
The decision to convert its status back to a mosque was made by the Council of State, Turkey's highest administrative body, with the backing of President Recep Tayyip Erdogan.
From the window of his apartment overlooking St Peter's Square, Pope Francis said he was thinking about Istanbul today.
"I am thinking of St Sophia and I am deeply pained," he said.
The World Council of Churches (WCC) has also criticised the decision. Its acting general secretary Rev Prof Dr Ioan Sauca has warned that the site could "become once again a focus of confrontation and conflict".
In a letter to Erdogan, he expressed the WCC's "grief and dismay" over the move, and urged the Turkish president to restore Hagia Sophia "to the emblematic unifying role" it has had as a museum.
"By deciding to convert the Hagia Sophia back to a mosque you have reversed that positive sign of Turkey's openness and changed it to a sign of exclusion and division," he said.
"Regrettably, this decision has also been taken without prior notice or discussion with UNESCO regarding the impact of this decision on Hagia Sophia's universal value recognized under the World Heritage Convention."
He continued: "The decision to convert such an emblematic place as Hagia Sophia from a museum back to a mosque will inevitably create uncertainties, suspicions and mistrust, undermining all our efforts to bring people of different faiths together at the table of dialogue and cooperation.
"Moreover, we greatly fear that it will encourage the ambitions of other groups elsewhere that seek to overturn the existing status quo and to promote renewed divisions between religious communities."
He concluded: "In the interests of promoting mutual understanding, respect, dialogue and cooperation, and avoiding cultivating old animosities and divisions, we urgently appeal to you to reconsider and reverse your decision."