Pope Benedict's visit to the Haghia Sophia will be closely watched
Pope Benedict XVI will take in a range of sensitive sites and subjects on his four-day trip to Turkey.
He will become only the second pontiff to visit a mosque on his visit to the mainly Muslim country, amid protests against his recent comments about Islam.
The head of the Roman Catholic Church will also meet Islamic and Jewish leaders as well as the heads of Turkey's Christian communities.
And Benedict will see the prime minister and president of the secular state, at a time of sensitivity there about the attitude of the Christian West towards Turkey, with talks on its EU membership bid on a knife-edge.
But correspondents say no moment may be more closely watched than his trip to the Hagia Sophia - a site heavy with symbolism.
The building's history spans Istanbul's evolution from Christianity to Islam - and is now a secular building, a museum.
Turkish Prime Minister Recep Tayyip Erdogan met the Pope when he arrived in Ankara.
John Paul II is the only pontiff to have visited the Blue Mosque
Mr Erdogan, whose party has Islamic roots, had not originally planned to see Benedict during his visit.
The Pope drew condemnation from across the Muslim world in September when he quoted the words of a Byzantine emperor who fought the Ottoman Turks and linked Islam to violence.
He is also seen by some as anti-Turkish for comments he made as a cardinal in which he appeared to oppose Turkey's EU membership bid.
The Vatican now says it is not opposed to such a move.
Patriarch Bartholomew I, spiritual leader of the world's Orthodox Christians, has reportedly said he will tell the Pope the EU must not be a "Christian club" and Turkey must be allowed to join.
'Paroxysms of fury'
The four-day visit was arranged with a view to improving relations with the Orthodox Church.
The Church split with the Vatican nearly 1,000 years ago over disputes including papal authority.
The meeting between the leaders of the two Churches is to be celebrated on 30 November, a feast day of huge importance to the Orthodox - that of their patron, Saint Andrew the Apostle, who brought Christianity to Asia Minor.
It is also of great significance for Pope Benedict, the BBC's David Willey reports from Rome.
Benedict says one of the long-term aims of his pontificate is to try to help heal the wounds of bitter theological disputes which have divided Christendom.
But in a visit heavy with religious symbolism, Pope Benedict also plans a trip to the famous 17th Century Blue Mosque.
His predecessor, Pope John Paul II, is the only other pontiff to have visited a mosque, during a trip to Damascus in 2001.
Benedict will also enter the Hagia Sophia. For 1,000 years the site was the largest church in Christendom, before it was converted into a mosque after the fall of Constantinople in 1453, and transformed into a museum in 1935, where religious services are banned.
Correspondents say an open act of piety by the Pontiff may enrage powerful Turkish nationalists, as a perceived signal of Christian claims to the site and a challenge to Turkish sovereignty.
"The risk is that Benedict will send Turkey's Muslims and much of the Islamic world into paroxysms of fury if there is any perception that the Pope is trying to re-appropriate a Christian centre that fell to Muslims," said an
editorial in Turkey's independent Vatan newspaper on Sunday.
The Pope will be treading carefully.