Pope Benedict XVI, left, and Ecumenical Patriarch Bartholomew I join together on a balcony at the Ecumenical Patriarchate after reading the Divine Liturgy in the nearby Church of St. George in Istanbul, Turkey, early today. Benedict XVI is in the third day of his four-day visit to Turkey.

ISTANBUL, Turkey — Pope Benedict XVI called divisions among Christians a "scandal to the world" at a joint ceremony today with the spiritual leader of the Orthodox Christian church, which split from Catholicism nearly 1,000 years ago.

Reaching out to the world's 250 million Orthodox Christians is a centerpiece of Benedict's papacy. He has set the difficult goal of full unity between the two ancient branches of Christianity, which divided over disputes including the extent of papal authority.

"The divisions which exist among Christians are a scandal to the world," the pope said after joining Ecumenical Patriarch Bartholomew I to mark the feast day of St. Andrew, who preached across Asia Minor and who is believed to have ordained the first bishop of Constantinople, now Istanbul.

The pope also said all Christians should "renew Europe's awareness of its Christian roots, traditions and values, giving them new vitality."

Benedict also is expected to sharpen his calls for what the Vatican labels "reciprocity" — that Muslim demands for greater respect in the West must be matched by increased tolerance and freedoms for Christians in Islamic nations.

But too much pressure by the pope, who arrived in Istanbul late Wednesday, could risk new friction with Muslims after broad gestures of goodwill that sought to ease simmering Muslim anger over remarks by the pope on violence and the Prophet Muhammad.

A statement claiming to be from al-Qaida in Iraq denounced the pope's visit as part of a "crusader campaign" against Islam and an attempt to "extinguish the burning ember of Islam" in Turkey. Vatican spokesman, the Rev. Federico Lombardi, said the declaration — posted on several Islamic militant Web sites — shows the need for faiths to fight "violence in the name of God."

He said "neither the pope nor his entourage are worried."

Still, Turkish authorities took massive security precautions for the Istanbul stop, with thousands of police on the street and roads cleared of all traffic for the papal motorcade.