Turkey beefed up its security ahead of Pope Benedict XVI’s four-day visit to the country, which began on Tuesday.
At least 12,000 security personnel are being deployed in Istanbul to safeguard the pope during his visit.
Many Turks have made it clear the pope is not welcome in their country.
Some 20,000 demonstrators marched in the streets of Istanbul on Sunday, chanting “Allah is great” and displaying banners urging the Pope to stay at home.
They were protesting against comments the pope made last September in which he cited a medieval text which said that the Prophet Muhammad’s teachings brought the world evil.
The remarks sparked angry protests throughout the Muslim world. Turks are demanding he formally apologizes for the comments.
The Pope is scheduled to meet with Turkish Prime Minister Recep Tayyip Erdogan and with religious leaders.
Turkish citizen Mehmet Ali Agca, the would-be assassin of the late Pope John Paul, has asked to be freed from prison to meet the pope for a theological discussion. His request is likely to be turned down.
Agca is scheduled to be released from prison in 2010. He was found guilty of the attempted attack on Benedict XVI’s predecessor in 1981, and of killing a Turkish writer.