The decision by President Recep Tayyip Erdogan sparked deep dismay among Orthodox Christians. Originally a cathedral, Hagia Sophia was turned into a mosque after Istanbul's conquest by the Ottoman Empire but had been a museum for the last 86 years.

By Suzan Fraser, Associated Press

Pope Benedict XVI visited the Blue Mosque in Istanbul, becoming the second pope to enter a Muslim place of worship. He praised Islam and supported Turkey's bid to join the European Union during his four-day trip.