Regular religious services have not been held there since 1934 when modern Turkey's founding president Mustafa Kemal Atatürk signed an order converting the building into a museum.
But a legal challenge claims the signature was forged.
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Turkish president joins hundreds of worshippers after iconic Istanbul landmark's controversial reconversion into mosque.
Turkey's highest court is set to decide the status of the Hagia Sophia, but experts say a legal ruling is only symbolic.
