The head of the World Council of Churches has written to President Recep Tayyip Erdogan to express his "grief and dismay" over Turkey's decision to change the status of Istanbul's landmark Hagia Sophia from a museum to a mosque. As a World Heritage site, "Hagia Sophia has been a place of openness, encounter and inspiration for people from all nations," interim secretary general Ioan Sauca said in the letter from the Geneva-based group on Saturday. The Hagia Sophia was built 1,500 years ago as an Orthodox Christian cathedral and was converted into a mosque after the Ottomans conquered Constantinople, now Istanbul, in 1453. The secular Turkish government decided in 1934 to make it a museum that millions of tourists now visit annually. Mr Erdogan formally redesignated the building as a mosque on Friday and declared it open for Muslim worship hours after a high court annulled the 1934 decision. Mr Sauca said museum status was "a powerful expression" of Turkey's commitment to inclusion and secularism. He urged Mr Erdogan to reconsider the decision "in the interests of promoting mutual understanding, respect, dialogue and cooperation, and avoiding cultivating old animosities and divisions". The council said its membership comprised 350 Protestant, Orthodox and Anglican churches with about 500 million believers. Mr Erdogan on Saturday rejected worldwide condemnation of Turkey's decision to convert the Byzantine-era monument back into a mosque, saying it represented his country's will to use its sovereign rights. He had repeatedly called for the Hagia Sophia to be rededicated as a mosque and, in 2018, he arranged for a reading from the Quran there. "Those who do not take a step against Islamophobia in their own countries ... attack Turkey's will to use its sovereign rights," Mr Erdogan said. The president's announcement came after a court overturned a decision made by modern Turkey's secularising founder Mustafa Kemal Ataturk to repurpose the Hagia Sophia as a museum. The court ruled that "there are no provisions whatsoever in the convention [concerning the Protection of the World Cultural and Natural Heritage] that prevents ... the usage of the Hagia Sophia in accordance with domestic law." Mr Erdogan then signed a decree handing the control of the Hagia Sophia mosque to Turkey's religious affairs directorate, Diyanet. "We made this decision not looking at what others say but looking at what right is and what our nation wants, just like we have done in Syria, in Libya and elsewhere," the Turkish leader said on Saturday.