The first Friday prayer in 86 years is being held at the Hagia Sophia in Istanbul.
It is expected that the Muslim prayer in the now-mosque Hagia Sophia will bring together over 1,500 people.
The Governor of Istanbul, Ali Yerlikaya, posted on Twitter a photo of the interior of the Hagia Sophia.
On 10 July Turkey's highest administrative court, the Council of State, annulled the decree converting Hagia Sophia into a museum. The move was poorly received abroad. The UNESCO world heritage site had been the largest Christian cathedral in the world for hundreds of years following its construction under Byzantine Emperor Justinian I in the sixth century.
Austria, France, Greece, Cyprus, Russia and the United States were among the countries that expressed regrets over Ankara's decision, while Turkey views the matter as its internal affairs.