Vatican City, Dec 25 (DPA) Pope Benedict XVI late Thursday led the Vatican's Christmas celebrations with the traditional midnight Mass, which this year began two hours earlier than usual.

Thousands of people flocked to St Peter's Basilica as Benedict began presiding over the ceremony. Many more followed the proceedings on giant video screens in St Peter's Square on a mild but damp winter's night.

Earlier this month the Vatican denied Italian media speculation that the decision to begin the mass at 10 p.m. (2100 GMT) was due to unspecified 'health problems' afflicting the 82-year-old pontiff.

At the time, papal spokesman Father Federico Lombardi explained that the move - first announced by the Vatican in October - aimed to 'tire the Pope a bit less,' giving him a few extra hours sleep before his Christmas Day duties.

In his Mass homily, Benedict was expected to stress the need for people to abandon selfishness and show a willingness to accept God - an attitude, according to the pontiff, exemplified by the shepherds who in the fields near Bethlehem are informed by an angel of Jesus' birth.

Earlier Thursday the Vatican unveiled in St Peter's Square its Nativity Scene, or crib, that recreates the scene of Jesus' birth - a custom revived in 1982.

The Gospel story narrates how Jesus' mother, Mary, and foster father Joseph, unable to find lodging, seek a manger for shelter where Christ is then born.

Several of the statues used in the Nativity Scene, some of them larger-than-life-size, were created in 1842 by St Vincent Pallotti for a crib in the Sant'Andrea della Valle church in Rome.

The Nativity Scene stands next to the Vatican's Christmas tree - this year a 30-metre high spruce from the forests of the Ardennes in Wallonia, Belgium.

On Friday Benedict is scheduled to deliver his Christmas Day blessings and traditional 'to the city and the world,' Urbi et Orbi message.