The nativity scene, or crib, has been a regular Christmas fixture in St Peter's since 1982, when the custom of building a four-metre-high house with lifesize figures in it was started by John Paul II.
Along with the giant fir tree which is always placed near it in the square, the crib has become a trademark of Roman Christmases and families often make special trips to see it.
Carpenters on Tuesday began building the temporary two-floor wooden house, in and around which the figures of Mary, Joseph, shepherds and wise men will be arranged. Electricians will later adorn the scene with lights.
During his papacy, John Paul made a habit of holding a special ceremony to inaugurate the St Peter's nativity scene in the last few days before Christmas. The Vatican has not yet announced whether Benedict will continue this tradition.
Elaborately decorated cribs, sometimes containing dozens of figures of people and animals, are a standard Christmas feature in millions of Italian households.
There are 17 figures in the Vatican's version, nine of which are the original statues made for a Roman church in 1842. Other minor figures have been added since. One of the latest, donated by Croatia in 2002, is of a
woman carrying a bread basket. Nativity scenes were the cause of polemics in Italy last year after several schools in the north decided against having one in order to avoid offending Muslim pupils.
Amid the controversy, John Paul II stressed that they were part of Christian culture.
"They are a familiar and expressive representation of Christmas. They are an element of our culture and our art," he said.
In the next few weeks the Christmas tree which will stand near the crib will also be erected. The giant fir,
which last year was 35 metres high, is usually a gift from an Italian or European region.
