At the Vatican, Pope Francis urged the world's 1.2 billion Catholics to feel compassion for children.
At the Vatican, Pope Francis urged the world's 1.2 billion Catholics to feel compassion for children, notably victims of war, migration and homelessness in his Christmas Eve mass.
Addressing a 10,000-strong crowd late Saturday in St. Peter's Square, the pontiff urged worshippers to think of the children "hiding underground to escape bombardment", in apparent reference to Syria.
In Bethlehem, 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 the Israeli-occupied West Bank near Jerusalem.
Like Pope Francis, Archbishop Pierbattista Pizzaballa also used his homily 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 at the mass, which was attended by Palestinian leader Mahmud Abbas and other dignitaries.
"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."
Security was tight across Israel where Christmas coincided with the Jewish festival of Hanukkah.
Prepared remarks by the leader of the world's Anglicans noted 2016 had left the world "more awash with fear and division".
"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," Justin Welby, the Archbishop of Canterbury, was to say in his Christmas Day sermon on Sunday.
In Europe, many preparing to celebrate were still reeling from Monday's truck attack on the Berlin Christmas market.
German authorities were working through the holiday season hunting possible accomplices to Tunisian Anis Amri, who was killed Friday in a shoot-out with Italian police near Milan.
Amri, 24, is believed to have hijacked a truck and used it to mow down holiday revellers at the market on Monday, killing 12 people in an attack claimed by the Islamic State group.
Tunisia said Saturday it had arrested three men suspected of links with Amri, including his nephew.
Locals and tourists in Berlin visited the Christmas market targeted in the attack, and many took a moment to quietly light a candle or lay flowers for the victims.
"It's really nice there are so many people here and it's still open," said Marianne Weile, 56, from Copenhagen.
"So even though you are really sad about what happened you can still keep Christmas. It's not like this crazy guy ruined it for everybody."
Security was also tight at Milan's cathedral, where Italian police were out in force and concrete barricades were erected around the historic Piazza del Duomo.
In France, 91,000 police, gendarmes and soldiers were deployed to guard public spaces including churches and markets.
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.
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.
Meanwhile, in debt-ridden Greece, Finance Minister Euclide Tsakalotos sent Christmas cards featuring the tight-fisted Dickensian protagonist of "A Christmas Carol", Ebenezer Scrooge, in a jibe to the country's creditors.
Christians in Syria's Aleppo were preparing for celebrations after President Bashar al-Assad's forces retook full control of the city following a rebel withdrawal this week.
Members of Aleppo's Catholic minority have been prepping for the first Christmas mass in five years at the Old City's Saint Elias Cathedral, whose roof collapsed under a salvo of rocket fire.
"All our memories are here -- this is where we celebrated all our feast days, our joys," said Bashir Badawi, rummaging through rubble for wood and scrap metal to make a crude Nativity scene.
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 who seized it in 2014.
"I can never describe... our happiness and everything. We feel like life returned," said Nada Yaqub.
The patriarch of Iraq's Chaldean Catholic Church, Louis Raphael Sako, urged international protection for Christians displaced by war so they could return to their homes.
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.
On the east coast, authorities evacuated thousands of people and shut down dozens of ports as a strong typhoon threatened to slam into the area later on Christmas Day before moving to the main island of Luzon.
In Britain, Queen Elizabeth II was due to pay tribute to "unsung heroes", in her annual Christmas Day broadcast.
"On our own, we cannot end wars or wipe out injustice, but the cumulative impact of thousands of small acts of goodness can be bigger than we imagine," the text of her speech read.
US President Barack Obama and first lady Michelle Obama, meanwhile, sent their final Christmas salutations from the White House on Saturday, highlighting common values uniting Americans of all faiths.