Pope Francis arrives to preside over the Christmas Eve mass at St. Peter's Basilica in the Vatican on Sunday. AFP

Pope Francis has kicked off global Christmas celebrations with a call for peace, as Israel's war on Hamas and Russia's invasion of Ukraine cast a shadow over one of the world's favourite holidays.

Having said earlier in the day that he was thinking of people "who are suffering from war — we are thinking of Palestine, of Israel, of Ukraine", the pope struck a sombre tone during his Christmas Eve mass.

"Tonight, our hearts are in Bethlehem, where the Prince of Peace is once more rejected by the futile logic of war, by the clash of arms that even today prevents him from finding room in the world," the pope said.

The biblical city in the occupied West Bank, where Christians believe Jesus Christ was born in a stable more than 2,000 years ago, effectively cancelled the annual Christmas celebrations that normally draw thousands of tourists.

The town did away with its giant Christmas tree, marching bands and flamboyant nativity scene this year, settling for just a few festive lights.

New Christmas Day

Ukraine, invaded by Russia nearly two years ago, is celebrating Christmas on December 25 for the first time, jettisoning the traditional Orthodox date of January 7, which is feted in Russia, as a snub to Moscow.

Pope Francis holds a statue of Baby Jesus as he presides over Christmas eve Mass at the Vatican, on Sunday. AP

In the southern Black Sea port of Odesa, churchgoers prayed and lit candles as priests in gold vestments held a Christmas Eve service in the Cathedral of the Nativity, decorated with fir trees and a nativity scene.

"We believe that we really should celebrate Christmas with the whole world, far away, far away from Moscow. For me that's the new message now," said one smiling parishioner, Olena, whose son is a medic on the front line.

The date change — moving away from the Julian calendar favoured by the Orthodox Church — is part of moves since the invasion to remove traces of the Russian and Soviet empires.

Agence France-Presse