The 80-year-old Argentine called for guns to fall silent in Syria.
Pope Francis urged peace in the Middle East as tens of thousands gathered to hear his Christmas address Sunday, while offering comfort to victims of terrorism after a year of bloody jihadist attacks.
The 80-year-old Argentine called for guns to fall silent in Syria, saying "far too much blood has been spilled" in the nearly six-year conflict.
And he urged Israelis and Palestinians to "have the courage and the determination to write a new page of history" in his message from the balcony of Saint Peter's Basilica to a crowd of 40,000 gathered in the square below which, despite the sunny weather, was far from full.
As Europe ramped up security for the holiday just days after the truck attack that left 12 dead at a Berlin Christmas market, the leader of the world's 1.2 billion Catholics said he hoped for "peace to those who have lost a person dear to them as a result of brutal acts of terrorism".
In Milan, where suspected Berlin attacker Anis Amri was killed in a police shootout on Friday, there was a heavy police presence around the cathedral. The entrance has been protected by concrete barriers since the Berlin attack.
In France, where Berlin has raised grim memories of the jihadist truck rampage in June that left 86 people dead, 91,000 members of the security forces have been deployed to guard public spaces including churches and markets over the weekend.
Religious ceremonies in Germany were heavy with the weight of Monday's attack, which was claimed by the Islamic State group.
"Christmas this year carries a deep wound -- we are celebrating this festival in a different way this year," said Gebhard Fuerst, bishop of Rothenburg in the southeast.
But Baden bishop Jochen Cornelius-Bundschuh offered a note of hope.
"At Christmas, a light shines in the world -- it shines in powerful darknesses like those we have seen in recent years with the horror of war, civil war and terrorist attacks," he said.
'Closed doors, defended borders'
In Israel, security was tight for Christmas celebrations coinciding with the Jewish festival of Hanukkah.
Some 2,500 worshippers packed the Church of the Nativity complex, built over the grotto where Christians believe Jesus was born, for midnight mass in Bethlehem in the Israeli-occupied West Bank.
Archbishop Pierbattista Pizzaballa used his homily there to plead for compassion for refugees and for a halt to the violence wracking the Middle East.
"We fear the stranger who knocks at the door of our home and at the borders of our countries," he said.
"Closed doors, defended borders, before personal and political choices, are a metaphor for the fear that inevitably breed the violent dynamics of the present time."
Pope Francis struck a similar tone in his Christmas Eve mass, urging a 10,000-strong crowd in St. Peter's Square to feel compassion for children, notably victims of war, migration and homelessness.
Justin Welby, the Archbishop of Canterbury who leads the world's Anglicans, meanwhile said 2016 had left the world more divided and fearful.
"The end of 2016 finds us all in a different kind of world; one less predictable and certain, which feels more awash with fear and division," he was due to say in his sermon Sunday.
Icy swim, meat auction
Queen Elizabeth II missed the Christmas Day church service attended by the British royal family as she suffered from a heavy cold, Buckingham Palace said.
The 90-year-old, who is the supreme governor of the Church of England, will join in the family festivities later in the day.
In London, meat-lovers converged on Smithfield Market for the traditional Christmas Eve auction at butcher Harts, waving banknotes in the air as they bid on turkeys, pork cuts and rump steaks.
Elsewhere in the world, despite the security fears, many were braving winter temperatures to take part in traditional revelry.
Among them some 30 hardy Slovaks participated in a winter swim at Bratislava's Zlate Piesky lake, some drinking beer in the nearly freezing water.
But in the world's conflict-torn countries, there were reminders of the violence that has ravaged the world this year.
Christians in Syria's Aleppo were preparing for Christmas services after President Bashar al-Assad's forces retook full control of the ruined former economic hub.
The Old City's Saint Elias Cathedral, its roof collapsed under rocket fire, was set to host its first Christmas mass in five years.
And in Bartalla, near the Iraqi city of Mosul, Christians filled the pews of the fire-scarred Mar Shimoni church for the first service since the town was retaken from IS jihadists who had seized it in 2014.
In the mostly Catholic Philippines, a blast ripped through a police car outside a church as worshippers were arriving for a Christmas Eve mass south of Manila, injuring 13 people.