Roman Catholics began filling up St. Peter's Square early Sunday, positioning themselves for the best spot to witness the historic canonization of two popes. As many as one million people are expected in St. Peter's and the nearby streets of Rome for the elevation-to-sainthood ceremony.

Churches throughout Rome were opened Saturday night, filled with pilgrims from around the world who have come to witness the canonization at the Vatican Sunday of two 20th century popes -- John the 23rd and John Paul the Second.

It is not immediately clear what role retired pontiff Benedict the 16th will play Sunday. His presence along with current Pope Francis sets the stage for the unprecedented occurrence of two living popes canonizing two of their predecessors.

The two being canonized are widely seen as representing contrasting factions of the Roman Catholic Church.

John, an Italian also known as the "Good Pope" because of his friendly, open personality, died before the Second Vatican Council ended its work in 1965, but his initiative set off one of the greatest upheavals in church teaching in modern times. The Council ended the use of Latin at Mass, brought in the use of modern music, and opened the way for challenges to Vatican authority, which alienated some traditionalists.

John Paul continued some of the reforms but tightened central control, condemned theological renegades and preached a stricter line on social issues. Groups representing victims of sexual abuse by Catholic priests say he did not do enough to root out a scandal that emerged towards the end of his pontificate and which has hung over the church ever since.

Both canonizations have involved some intervention with the normally strict rules governing the declaration of a saint. Francis ruled that only one miracle was needed to declare John a saint, while Benedict waived a rule that normally requires a five-year waiting period before the preliminaries to sainthood can even begin to speed up John Paul's canonization.