Pope Francis called for peace and "fraternity" and Queen Elizabeth II warned against "tribalism", as millions across the world celebrated Christmas on Tuesday -- after US President Donald Trump caused a festive furore by raising doubts about Santa Claus's existence during a call with a child.

Some 50,000 worshippers gathered at the Vatican to hear the pontiff's sixth "Urbi et Orbi" (To the City and the World) message appealing for peace in conflict zones such as Syria and Yemen, which face humanitarian crises.