Friday prayers are held at the Hagia Sophia mosque in Istanbul for the first time since it was turned into a museum 85 years ago.
Only around 1,000 people are allowed in at one time due to the coronavirus.
Turkish President Recep Tayyip Erdogan is among the faithful.
A large screen and speakers have been installed outside to broadcast the proceedings to the large crowds gathered outside.
It was at the instigation of President Erdogan that a Turkish court reversed a decision made in the 1930s to make it a secular building.
Supporters of the president celebrate the measure as a moment of great national and religious pride. However, his critics fear he is trying to replace the secular legacy of Turkey’s modern founder, Kemal Ataturk.
Hagia Sophia, which was built 1,500 years ago, was an Orthodox Christian cathedral until the Ottoman conquest in the 15th century. Protests are planned in the Greek capital, Athens, later on Friday against her conversion into a mosque.
As the crowd grew, authorities detained people who entered the area. On Twitter, Istanbul Governor Ali Yerlikaya urged worshipers to be patient and said the mosque would remain open for prayers until Saturday morning.
.