Pope Francis stood in silence for several minutes at his weekly Angelus prayer Sunday, after he talked about Turkey's decision to convert the Hagia Sophia back into a mosque.
“The sea takes my thought a bit far, to Istanbul,” the pontiff said during the prayer, which commemorates people who work at sea.
“I think of Hagia Sophia, and I am very saddened.”
Since a cabinet decree in 1934, the iconic building in Istanbul has served as a museum, but on Friday, Turkish President Recep Tayyip Erdoğan issued a presidential decree to transfer the management of the building from the Ministry of Culture to the Presidency of Religious Affairs.
The order follows a previous ruling by a Turkish court that annulled the decades-old cabinet decree.
The Hagia Sophia is a UNESCO World Heritage site, and the group says it deeply regrets Turkey's decision.
“Hagia Sophia is an architectural masterpiece and a unique testimony to interactions between Europe and Asia over the centuries,” said the Director-General Audrey Azoulay.
“Its status as a museum reflects the universal nature of its heritage, and makes it a powerful symbol for dialogue.”
Greece and France have also called into question Turkey's secularism.
The iconic building's history stretches back to the Byzantine empire in the 6th century, when it was constructed as a cathedral. In the 15th century, it became a mosque.
Since Islam forbids images of people to be displayed in mosques, officials plan to use lights and curtains to cover up the numerous Byzantine Christian mosaics in the Hagia Sophia, Turkey's head of religious affairs Ali Erbaş said Sunday on Turkish TV.
After prayer, the paintings will be uncovered for visitors once again. Entrance fees will also be canceled, President Erdoğan said.
“Like all our mosques, its doors will be open to everyone — Muslim or non-Muslim,” Erdoğan said.
“As the world's common heritage, Hagia Sophia, with its new status, will keep on embracing everyone in a more sincere way.”
CNN's Sharif Paget, Murat Baykara, Isil Sariyuce, Livia Borghese and Zahid Mahmood contributed to this report.
