Security forces are on full alert for the pope's visit. Nearly 4,000 police, including units in full riot guard, watched over the protest. Surveillance helicopters buzzed overhead and protesters were frisked before entering the square in a conservative stronghold of Istanbul.

The pope's visit has two distinct - and difficult - objectives: calming Muslim ire and advancing efforts to heal a nearly 1,000-year divide in Christianity between the Vatican and Orthodox churches.

Benedict plans to meet first with political and Muslim religious leaders in the capital, Ankara, including Turkey's president and the Islamic cleric who oversees the country's religious affairs. Prime Minister Recep Tayyip Erdogan is scheduled to attend a NATO meeting in Latvia during the papal visit, but could briefly greet the pontiff at the airport.

The pope then heads to Istanbul - the ancient Byzantine capital of Constantinople - to be hosted by the spiritual leader of the world's Orthodox Christians, Ecumenical Patriarch Bartholomew I.

The pope strongly backs efforts for closer bonds between the two ancient branches of Christianity, which formally split in the 11th century over disputes including papal primacy. But some Orthodox leaders, including Russia's powerful Patriarch Alexy II, are wary of deepening ties too fast.

While in Istanbul, Benedict also plans to visit the famous 17th century Blue Mosque. The Vatican spokesman, the Rev. Federico Lombardi, called it a "sign of respect" toward Muslims.

The mosque is one of the city's major tourist sites and its slender minarets are a prominent landmark in Istanbul's ancient center. Tradition says it was built to show Islamic architects could rival the glories of the nearby Haghia Sophia, a church that was converted to a mosque after the city fell to Muslim armies in 1453. It is now a museum.

Benedict may also use his time in Turkey as a forum to demand that Islamic nations offer greater rights and protection to Christian minorities, such as the remnants of the once-thriving Greek Orthodox community in Istanbul.

1

2

3