Istanbul's Hagia Sophia, which means "Holy Wisdom" in Greek, was completed in 537 by Emperor Justinian and considered the center of Orthodox Christianity for more than 900 years. It was converted in the mid-15th century into a mosque after conquest by the Ottomans and became a source of pride for the Muslim world.

"Keeping Hagia Sophia Mosque closed is an insult to our mostly Muslim population of 75 million. It symbolizes our ill-treatment by the West," said Salih Turhan, head of the Anatolian Youth Association.

There were also some Greek Orthodox Christians living in Turkey who believe the museum should re-assume its function as a church. However, the Orthodox patriarchate disagrees.

"If it were to become a mosque, Christians wouldn't be able to pray there, and if it became a church it would be chaos," Father Dositheos Anagnostopulos, a spokesman for the patriarchate, told Reuters.

In 2006, Pope Benedict XVI's visit to Hagia Sophia was met by protests from angry Muslims who saw the pontiff's presence as an attempt to make a Catholic claim of the site.

The demonstrators warned that any sign of a prayer there would be considered an offense, BBC News reported, noting that Benedict abstained from making any religious gesture.

According to latest estimates, Turkey's population, numbering around 79.7 million, is 99.8 percent Muslim, while the remaining 0.2 percent are mostly Christians and Jews.

According to Reuters, just a few thousand Greek Orthodox Christians remain in Turkey, although the patriarch's seat is still in Istanbul, as it was before the Ottoman conquest.