Pilgrims from around the world are flocking into Saint Peter's Square ahead of Pope Benedict's first Christmas Eve midnight Mass since he assumed the papacy in April.

Thousands will crowd into Saint Peter's Basilica for the service marking the opening of celebrations of the feast of the birth of Jesus Christ, whom Christians call the Prince of Peace. Crowds of others will watch the Mass on huge television screens in the square.

Ahead of the Mass, the pope appeared at his study window and joined the pilgrims in singing Christmas carols during a prayer service at a life-sized nativity scene in the square.

At noon Sunday, Christmas Day, Pope Benedict will deliver his first "Urbi et Orbi" (To the City and the World) message as well as his blessings and greetings to the faithful of the world from a balcony overlooking Saint Peter's Square.