The head of the Catholic Church denounced the continuing suffering of children.
He mentioned those who faced hunger, danger on migration routes and bombing in Syrian cities such as Aleppo.
Those taking part in the service inside St Peter's Basilica had to pass through metal detectors.
Saturday's Mass celebrating the birth of Jesus is the first major event of the Christmas season for the Pope, which will include his noon blessing on Christmas Day.
During the service, he said Christmas had been "taken hostage" by materialism, and that the season needed more humility.
Throughout the past year, the Pope has urged compassion towards refugees, urging Christians to remember that Jesus himself was a migrant.