Unexpected meeting between Francis and Sheikh Ahmed al-Tayeb followed easing of tensions during reign of Benedict.
Pope Francis has urged an end to the fighting in Syria as he gave his Christmas address at the Vatican, saying “far too much blood has been spilled” in the conflict.
His traditional “Urbi et Orbi” (To the City and The World) message on Sunday was linked by a common theme of war, violence and suffering.
“It is time for weapons to be still for ever, and the international community to actively seek a negotiated solution,” he told some 40,000 people gathered in St Peter’s Square.
Francis, marking the fourth Christmas season since his election in 2013, also urged Palestinians and Israelis, facing renewed tension after a UN resolution condemning Israeli settlements on occupied land, to have the courage to put aside hate and revenge and “write a new page of history”.
“Peace to those who have lost a person dear to them as a result of brutal acts of terrorism, which have sown fear and death into the hearts of so many countries and cities,” he said.
Security was heightened for the Christmas weekend in Italy and at the Vatican after Italian police killed the man believed to be responsible for the Berlin market truck attack while other European cities kept forces on high alert.
“Today this message [of peace] goes out to the ends of the earth to reach all peoples, especially those scarred by war and harsh conflicts that seem stronger than the yearning for peace,” he said, speaking in Italian from the central balcony of St Peter’s Basilica.
He called for immediate assistance to the exhausted population of the city of Aleppo, which Syrian government forces recaptured last week after four years of devastating fighting with rebels.
Members of the Christian minority community gathered at St Elias Cathedral in Aleppo’s old city, as prayers were held for peace at the first Christmas Eve Mass for five years.
“The festive atmosphere is great. It’s a new birth for Jesus Christ and a new birth for the city of Aleppo,” George Bakhash, a Christian community leader, told Reuters news agency.
Francis, the first Latin American pope, also said Christmas should inspire everyone to help the less fortunate, including migrants, refugees and those swept up by social and economic upheavals.
“Peace to the people who suffer because of the economic ambitions of the few, because of the sheer greed and the idolatry of money, which leads to slavery,” he said.
At his Christmas Eve Mass on Saturday, Francis said the feast had been “taken hostage” by dazzling materialism that puts God in the shadows.
On Sunday, he also called for an end to “fundamentalist terrorism” in Nigeria, a reference to Boko Haram, which has killed 15,000 people and displaced more than two million during a seven-year campaign to create an Islamic state.
Francis further appealed for an end to tensions between the government and opposition in Venezuela, for harmony in Colombia – which recently ended a civil war with FARC rebels – and an end to strife on the Korean peninsula and in Myanmar.