Pope Benedict XVI has met Ecumenical Patriarch Bartholomew I in Turkey, on the second day of a landmark visit to the largely Muslim country.
The two leaders held a joint prayer service in Istanbul
The Istanbul talks with the spiritual leader of the world's Orthodox Christians aimed to heal an old rift.
Earlier, the Pope held Mass near a shrine to the Virgin Mary in the ancient western city of Ephesus.
The four-day visit to Turkey has been overshadowed by comments the Pope made in September about Islam.
The meeting with Bartholomew - who heads a community of some 250 million Christians around the world - was the original reason for the pontiff's decision to travel to Turkey.
29 Nov: Goes to Ephesus to celebrate mass at site where Virgin Mary believed to have died Goes to Istanbul to meet Patriarch Bartholomew I, spiritual leader of the Orthodox Church, for first of series of encounters
1 Dec: Visits Haghia Sophia (6th Century Byzantine church converted into mosque in 1453, then transformed into museum in 1935) and Blue Mosque
The two leaders began their meeting by holding a joint prayer service at the St George Church in Istanbul.
The two ancient branches of Christianity - the Eastern and the Western rites - split nearly 1,000 years ago over disputes including papal authority.
In Istanbul, Benedict will also meet faith leaders and visit the city's famous Blue Mosque. He is also scheduled to lead Mass in a cathedral.
Istanbul was once Constantinople - the centre of the Byzantine empire, but now the largest city in a secular Turkish republic.
The service in Ephesus was the only open-air Mass Pope Benedict was to say in Turkey, for a congregation of some 500 Catholics brought to the shrine by special invitation.
The Pope visited the small stone house set in the lush green hillside where the Virgin Mary is thought to have spent her last days.
The Pope was in Ephesus to visit a shrine to the Virgin Mary
It is visited every year by tens of thousands of pilgrims.
He also honoured the memory of a Roman Catholic priest who was killed amid Muslim anger over the publication of caricatures of the Prophet Muhammad.
"Let us sing joyfully, even when we're tested by difficulties and dangers, as we have learned from the fine witness given by the Roman priest John Andrea Santoro, whom I am pleased to recall in this celebration," he said.
There is only a tiny Catholic community left in Turkey and many in the congregation were foreigners who live in the country and some had travelled from the Mediterranean coast for the occasion.
Call for dialogue
On Tuesday, Pope Benedict called for an "authentic dialogue" between Christians and Muslims in a speech at Turkey's directorate of religious affairs.
He said the exchange must be "based on truth and inspired by a sincere wish to know one another better".
The visit has been overshadowed by angry protests by Turkish Muslims.
Tens of thousands of people protested in Istanbul at the weekend, calling on the Pope to stay away or apologise for comments he made about Islam in a speech in September.
Speaking to an academic audience in Germany, the Pope quoted a Byzantine emperor who characterised Islam as a violent religion.
While the Pope insisted the remarks did not reflect his own views, the speech was widely reported and caused anger across the Islamic world.
WHERE EAST MEETS WEST
Kosovo is an overwhelmingly Muslim province of Serbia, pushing for independence
In Lviv and other western parts of Ukraine, the Ukrainian Greek Catholic Church predominates - a church that follows Eastern rites but vows allegiance to Rome
Republika Srpska is the Serb part of Bosnia
Cyprus is divided between the Greek, Orthodox south and the breakaway Turkish, Muslim north