Turkish President Recep Tayyip Erdogan will take part Friday in the first prayers at Hagia Sophia since the Istanbul landmark was reconverted to a mosque despite international condemnation.
A top Turkish court revoked the sixth-century monument's status as a museum on July 10 and Erdogan then ordered the building to reopen for Muslim worship, deeply angering the Christian community and further straining relations with NATO ally Greece.
The UNESCO World Heritage site in historic Istanbul was first built as a cathedral in the Christian Byzantine Empire but was converted into a mosque after the Ottoman conquest of Constantinople in 1453.
Some 3.8 million tourists visited the museum last year.
The Council of State, the highest administrative court, unanimously cancelled a 1934 decision by Turkey's modern founder Mustafa Kemal Ataturk to turn it into a museum, saying it was registered as a mosque in its property deeds.
The head of Turkey's religious affairs directorate, Ali Erbas, said on Wednesday that up to 1,000 people could take part in the Friday prayer, which will be preceded by a Holy Quran recital.
Leaders and officials from several Muslim-majority countries were invited, including Qatar and Azerbaijan, Turkish media reported.
Nearly 20,000 security forces will be in the area to ensure the first prayer takes place without incident.