Pope Francis arrived in Istanbul Saturday following his visit to Ankara amid tight security against any possible radical attacks.
Istanbul raised the security level to red as up to 5,000 policemen were positioned every 50 metres on the way between the airport and Istanbul's historical peninsula, where the Pope visited Sultanahmet mosque and the Hagia Sophia museum, Xinhua reported.
Turkey already has a bad reputation as Turkish Mehmet Ali Agca shot and wounded Pope John Paul II in the Vatican in 1982.
Pope Francis's visit to Hagia Sophia has been considered symbolically important.
After serving as a Greek Orthodox cathedral of Constantinople for 916 years, the site was then transformed into a mosque with the fall of Byzantine, but today it is a secular museum for all in Turkey.
The pope is later expected to celebrate mass in the Cathedral of the Holy Spirit in Sisli, the European district of Istanbul, mostly populated by non-Muslims.
He is also expected to meet Patriarch Bartholomew later Saturday in a move to develop better ties with the Orthodox churches. The Catholic and the Orthodox churches have been divided for more than 1,000 years.
Pope Francis's visit to Turkey is seen as part of his efforts to bridge different faiths amid the rampage by the Islamic State Sunni radical group in Iraq and Syria.
