Pope Francis on Sunday joined an international chorus of condemnation of Turkey's decision to convert Istanbul's iconic Hagia Sophia landmark back into a mosque.

"I think of Hagia Sophia, and I am very saddened," Pope Francis said towards the end of his midday sermon in Saint Peter's Square.

Muslims gathered outside the Hagia Sophia on Friday to celebrate the court decision that cleared the way to converting it back to a mosque Muslims gathered outside the Hagia Sophia on Friday to celebrate the court decision that cleared the way to converting it back to a mosque Photo: AFP / Ozan KOSE

It was the Vatican's first reaction to Turkey's decision to transform the Byzantine-era monument back into a mosque, a decision that has already drawn criticism from around the world.

The Vatican newspaper Osservatore Romano on Saturday carried reaction from different countries to Friday's decision to turn the monument from a museum back into a mosque, but without any comment.

A magnet for tourists worldwide, the Hagia Sophia was first constructed 1,500 years ago as a cathedral in the Christian Byzantine Empire and it was there they crowned their emperors.

Pope Francis made his comments about the Hagia Sophia at the end of his Sunday Angelus prayer Saint Peter's square Pope Francis made his comments about the Hagia Sophia at the end of his Sunday Angelus prayer Saint Peter's square Photo: AFP / Vincenzo PINTO

It was converted into a mosque after the Ottoman conquest of Constantinople in 1453, then became a museum in 1935.

Turkey's President Recep Tayyip Erdogan, who critics say is chipping away at the Muslim-majority country's secular pillars, announced Friday that Muslim prayers would begin on July 24 at the UNESCO World Heritage site.

Several other Christian leaders have already spoken out against Turkey's decision.

Some Greeks protested outside the Agia Sofia church of Thessaloniki, in northern Greece, when news broke of Turkey’s decision Some Greeks protested outside the Agia Sofia church of Thessaloniki, in northern Greece, when news broke of Turkey’s decision Photo: AFP / Sakis MITROLIDIS

Bishop Hilarion, who heads the Russian Orthodox Church's department for external church relations, described it as "a blow to global Christianity".

The World Council of Churches, which represents 350 Christian churches, said it had written to Erdogan expressing their "grief and dismay".

Istanbul's Hagia Sophia Istanbul's Hagia Sophia Photo: AFP / Robin BJALON

The head of the Greek Orthodox Church, Archbishop Ieronymos, on Sunday denounced what he described as the "instrumentalisation of religion to partisan or geopolitical ends".

"The outrage and the arrogance doesn't just concern the Orthodox Church and Christianity but all of civilised humanity... independently of religion," he added.

Greek Culture Minister Lina Mendoni took a similar view, calling Turkey's decision "a provocation to the civilised world".

Prime Minister Kyriakos Mitsotakis also condemned the move, not just for the damage it would do to relations between Greece and Turkey, but Ankara's relations with "the European Union, UNESCO and the world community".

The transformation of the Hagia Sophia dominated the headlines in Greek newspapers this weekend.

The Kathimerini newspaper stressed the political dimension of Turkey's decision, which it said effectively underlined the secular roots of modern Turkey and demonstrated "Erdogan's megalomania".

Erdogan on Saturday dismissed protests from Russia, the United States, France and UNESCO.

"Those who do not take a step against Islamophobia in their own countries ... attack Turkey's will to use its sovereign rights," he said.

In the past, he had repeatedly called for the stunning building to be turned back into a mosque and in 2018, he recited a verse from the Koran at the Hagia Sophia.

Erdogan's announcement came after a top court cancelled a 1934 cabinet decision under modern Turkey's secularising founder Mustafa Kemal Ataturk to preserve the then church-turned-mosque as a museum.