Visitors wearing protective face masks take a 'selfie' smartphone photograph outside the Hagia Sophia museum  in Istanbul. (PHOTO / BLOOMBERG)

ATHENS - After a gap of nearly a century, Istanbul’s iconic Hagia Sophia on Friday echoed with the sounds of Muslim prayer, with Turkey's President Recep Tayyip Erdogan joining the faithful to witness the fulfillment of a move calculated to boost his popularity at home and in the broader Muslim world.

Friday’s service is the first since Erdogan’s decision earlier this month to act on a court order he had encouraged and convert the gracious domed structure originally built as a Byzantine cathedral from a museum to a mosque.

In sharp contrast to the international criticism that greeted the decision, the president’s followers reacted with joy. Hundreds of men slept outside Hagia Sophia from late Thursday, reciting prayers and chanting hymns as they waited for the grand opening.

Hundreds of meters of turquoise carpets were laid on the marble floor while curtains were installed to cover the early Christian mosaics that line the great dome.

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Erdogan’s Islamist-rooted ruling AK Party has lost ground in Turkey’s major cities in recent years, and the president hopes to rally the support of nationalist Turks by portraying the monument’s new status as a milestone in the country’s rebirth as a powerful Muslim nation after a century of misguided efforts to imitate the Christian West.

But the move is likely to damage already difficult relations with European neighbors led by Greece, for which the Hagia Sophia carries a special significance as one of the most important monuments of Orthodox Christianity.

Church bells across Greece tolled in mourning on Friday as the first Muslim prayers in nine decades were held at Hagia Sophia in Turkey, marking the monument’s conversion into a mosque.

Greek criticism of the conversion has been scathing, underscoring often tense relations between Greece and Turkey.

In a message marking Greece’s 46th anniversary of the restoration of democracy on Friday, Greek Prime Minister Kyriakos Mitsotakis called Turkey a ‘troublemaker’, and the Hagia Sophia conversion an ‘affront to civilisation of the 21st century’.

Inside the building’s security perimeter, hasty preparations have made it suitable for Muslim worship for the first time since secular leader Mustafa Kemal Ataturk gave it museum status.

Hundreds of meters of turquoise carpets were laid on the marble floor while curtains were installed to cover the early Christian mosaics that line the great dome.

The first and only posting so far from a newly created Twitter account to mark the occasion read: “In the name of Allah the Merciful,” in Arabic letters. The expression, known as the Basmala, is said at the opening of Koranic chapters in order to receive blessing from God.

READ MORE: Turkey will inform UNESCO about Hagia Sophia moves, minister says

Why Now?

The Byzantine cathedral has been converted on two previous occasions in the past millennium, both of them momentous junctures for the region.

It was turned from a Christian church into a mosque after the Ottoman conquest of Constantinople, now Istanbul, in 1453 -- and from mosque to museum in 1934, as part of Ataturk’s efforts to secularize his new Turkish Republic.

The latest decision has been a long time coming. And like Ataturk’s museum conversion -- which helped smooth the path to a defense pact with Greece at a time of rising military threats in Europe -- the timing was political. Erdogan’s popularity has been sagging under the weight of a damaged economy, and conservative rivals who threaten to eat into his vote.

“He hopes this will be his legacy that he will be remembered with,” said Ozgur Unluhisarcikli, Turkish head of the German Marshall Fund of the United States, said. “And all right-wing voters support such a cause.”