Earlier, crowds formed at checkpoints around the historic heart of Istanbul where massed police maintained security. Once through the checks, worshippers sat apart on prayer mats in secured areas outside the building in Sultanahmet Square.

“We are ending our 86 years of longing today,” said one man Sait Colak, referring to the nearly nine decades since Hagia Sophia was declared a museum and ceased to be a place of worship. “Thanks to our president and the court decision, today we are going to have our Friday prayers in Hagia Sophia.”

“This is the opening of a place of worship that was conquered by the right of the sword by the holy conqueror,” said worshipper Latif Ozer, 42. “This is a source of great pride for us, great excitement.”

A top Turkish court announced this month it annulled Hagia Sophia’s status as a museum. Erdogan immediately turned back into a mosque a building which was a Christian Byzantine cathedral for 900 years before being seized by Ottoman conquerors and serving as a mosque until 1934.

As crowds grew, leaving little space for social distancing, Istanbul Governor Ali Yerlikaya said authorities had stopped people entering the area due to concerns about the spread of the coronavirus.

On Twitter he called for patience and said the mosque would be open for prayer until Saturday morning.

“God is greatest,” chanted people in the square. Some slept after arriving overnight and others ate on the grass, shaded by trees from the hot sun. Some in the crowds held Turkish and Ottoman flags.

After leaving Hagia Sophia, Erdogan went straight to the nearby Fatih (Conqueror) mosque, named after Sultan Mehmet who seized Istanbul for the Ottomans.

“Hagia Sophia will continue to serve all believers as a mosque and will remain a place of cultural heritage for all humanity,” Erdogan said at Mehmet’s tomb. “We said ‘let’s visit the tomb of Sultan Mehmet the Conqueror, the real owner”.

During his 17-year rule, Erdogan has championed Islam and religious observance and backed efforts to restore Hagia Sophia’s mosque status. He said Muslims should be able to pray there again and raised the issue – popular with many pious AKP-voting Turks – during local elections last year.

Why Hagia Sophia move spells trouble for Turkey’s President Erdogan

The conversion triggered fierce criticism from church leaders, who said the conversion to exclusively Muslim worship risked deepening religious divisions. Turkey says the site will remain open for visitors and its Christian artworks protected.

Erdogan has reshaped Turkey’s modern republic, established nearly a century ago by the staunchly secularist Mustafa Kemal Ataturk, lifting a ban on Muslim headscarves in public, promoting religious education and taming Turkey’s powerful military, once a bastion of Ataturk’s secular values.

Inside Hagia Sophia, the Christian frescoes and the glittering mosaics adorning the cavernous dome and central hall will be concealed by curtains during Muslim prayer times, but remain on display for the rest of the time.

Meanwhile, church bells across Greece tolled in mourning on Friday with their flags at half-mast to protest what the head of the Church of Greece, Archbishop Ieronymos, has called an “unholy act of defiling” of the former cathedral.

“(Today) is a day of mourning for all of... Christianity,” Ieronymos said.

Hagia Sophia is “a symbol of our faith and a universal monument of culture,” he said.

Greek Prime Minister Kyriakos Mitsotakis called Turkey a “troublemaker”, and the Hagia Sophia conversion an “affront to civilisation of the 21st century”.

Religious and nationalist groups will hold protests in Athens and Thessaloniki later Friday.

Additional reporting by Agence France-Presse