The pope, wearing white robes, stopped often to gaze on Quranic passages carved in the ancient marble — in some places where crosses and the fish-shaped sign of early Christians were chiseled away. Above them were frescoes and mosaics that couldn't be touched by Muslims: figures such as Jesus and the Virgin Mary, who are regarded as revered predecessors of Muhammad.

Security for the pilgrimage has been stringent. But it grew even tighter as the pope moved about Istanbul. Police blockades virtually sealed off parts of the city's ancient heart. Snipers stood watch on the minarets added to Haghia Sophia following the Muslim conquest.

About 150 nationalists demonstrated against the pope's visit to the site, gathering at a square about a half mile away and urging the government to open the museum to Muslim worship. Nationalists viewed the visit as a sign of Christian claims to the site and a challenge to Turkish sovereignty.

"Haghia Sophia is Turkish and will remain Turkish," one protest sign read. Riot police surrounded the demonstrators to prevent them from reaching the site.

Of Turkey's 70 million people, some 65,000 are Armenian Orthodox Christians, 20,000 are Roman Catholic and 3,500 are Protestant. Some 23,000 are Jewish.