President Recep Tayyip Erdogan on Sunday inaugurated the first church built with government backing in overwhelmingly Muslim Turkey's 100-year history as a post-Ottoman state.

The Mor Ephrem Syriac Orthodox Church's opening marks an important cultural and political moment for both Turkey and its powerful leader.

Erdogan drew widespread condemnation during his two-decade rule for converting ancient churches into mosques and making Islamic conservatism into a leading social force.

He has always countered that he was simply restoring the rights of pious Muslims in the staunchly secular republic founded by field marshal Mustafa Kemal Ataturk in 1923.

Erdogan laid the first stone for the church's construction for Istanbul's 17,000-strong Assyrian Christians in 2019.

"We are seeing big problems today across many parts of the world," Erdogan told the faithful as all-out war raged between Israel and the Palestinian militant group Hamas in Gaza.

"But the solidarity shown here today — I find it very important," Erdogan said.

"We always protect the oppressed against the oppressor. That is our duty."

Assyrian Christianity traces its history to communities that lived in the first century AD in a region stretching from southeastern Turkey to Syria and Iraq.

Its main church moved from the Turkish city of Mardin to Damascus in 1932.

'Love letter'

Some small Turkish churches have been quietly restored and re-opened in the past 100 years.

Erdogan said on Sunday that 20 existing churches had been repaired since his Islamic-rooted party came to power in 2002.

But the Mor Ephrem "is the first newly built church to open its doors since the founding of the Turkish Republic," Assyrian community leader Sait Susin told AFP by telephone.

"We are very happy."

Erdogan drew international indignation for converting Istanbul's iconic Hagia Sophia —once the world's largest cathedral —from a museum into a mosque in 2020.

The United Nations cultural body UNESCO expressed "grave concern" at the time.

Erdogan brushed the criticism aside and did exactly the same thing to Istanbul's Byzantine-era Chora Church later that same year.

Greece called that conversion "yet another provocation against religious persons everywhere".

Erdogan came under particularly strong attacks at home for unveiling a new mosque in 2021 on Taksim Square — an Istanbul gathering point built around a monument celebrating Ataturk's foundation of the secular Turkish state.

The new Istanbul church can accommodate 750 worshippers.

Erdogan wavers in his speeches between robustly defending pious Muslims and embracing Turkey's numerous communities.

He told supporters on the eve of the first round of May's presidential election that he had written a "love letter" to Turkey.

"We have penned a love letter for every individual of our nation, without any distinction of origin or religion," he told the crowd.

He ended that day by leading Muslim prayers at the Hagia Sophia mosque.

Erdogan edged out his secular rival in a runoff election two weeks later.