Pope Francis urged peace in the Middle East at his Christmas address Sunday, in which he offered comfort to terrorism victims after a year of brutal jihadist attacks.
The 80-year-old Argentine pontiff called for guns to fall silent in Syria, saying "far too much blood has been spilled" in the nearly six-year conflict which has also contributed to mass migration and homelessness.
And despite few signs of progress he urged Israelis and Palestinians to "have the courage and the determination to write a new page of history" in his message delivered from the balcony of Saint Peter's Basilica.
A crowd of 40,000 gathered in the square below which, despite the sunny weather, was far from full.
Europe has ramped up security for the holiday just days after the truck attack that left 12 dead at a Berlin Christmas market on Monday.
Francis, 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 the suspected Berlin attacker was killed in a police shootout on Friday, there was a heavy security presence around the cathedral. The entrance has been protected by concrete barriers since the Berlin atrocity.
In France, where the killings in Germany evoked memories of the jihadist truck rampage in June that left 86 people dead in Nice, 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 the country's interior minister Thomas de Maiziere praised the public's reaction as "brilliant".
"I am impressed by the determination... the Germans remain unshakeable," he told the Bild newspaper.
- 'Closed doors, defended borders' -
In Israel, security was tight for Christmas celebrations coinciding with the Jewish festival of Hanukkah.
Around 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 to plead for compassion for refugees and for a halt to Middle East violence.
"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 had struck a similar tone in his Christmas Eve mass, urging a 10,000-strong crowd to feel compassion for children, notably victims of war, migration and homelessness.
Justin Welby, the Archbishop of Canterbury who is the spiritual leader of the world's Anglicans, said 2016 had left the world less stable.
"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.
In The Netherlands, King Willem-Alexander called for national unity, pointing to unsettled politics, warning that the "extreme seems to have become the new normal".
"The contradictions in the world have become bigger," he said on national television.
- Icy swim, meat auction -
In Britain, a heavy cold meant Queen Elizabeth II missed the Christmas Day church service attended by the royal family.
The 90-year-old, who is the supreme governor of the Church of England, will join in the family festivities later in the day, Buckingham Palace said.
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 for turkeys, pork cuts and rump steaks.
Elsewhere in the world, despite security fears, many were braving winter temperatures to take part in traditional celebrations.
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.
