VATICAN CITY — St. Peter's Square will again be flooded with pilgrims and pageantry today when Pope Benedict XVI is installed as leader of the Roman Catholic Church at an outdoor Mass marked by both innovations and long-forgotten traditions.

While Cardinal Joseph Ratzinger was elected by the College of Cardinals as the 265th pope on Tuesday, and has made several appearances and speeches in his new white robes, the 78-year-old German formally takes the papal throne during today's ceremony of investiture.

Italian officials expect about 100,000 people from Germany alone to come to the Mass. Yesterday, Vatican workers laid grass on the steps of St. Peter's Basilica while Rome's civil-protection workers prepared for the huge crowds by lining guardrails along the Via della Conciliazione, the broad avenue that leads to the Vatican.

The two-hour ceremony, celebrated by the senior cardinal deacon, Jorge Arturo Medina Estevez, a Chilean who announced Benedict to the world, will have the familiar framework and rituals of a regular Sunday Mass.

But the Vatican said yesterday that there would also be some novelties.

Before the ceremony begins, the pope will for the first time make a solemn visit to what Catholics believe is the tomb of St. Peter, in the basilica's crypt. Waiting for him there will be the symbols of his papal authority, the Fisherman's Ring and pallium, a narrow stole of white wool.

The Rev. Crispino Valenziano, an expert on liturgical rites at the Vatican, said that officials from the Association of Rome's Goldsmiths fitted the pope for the Fisherman's Ring two days ago, and that when Benedict was told he wore a size 24 ring, he said, "I like 24, it's the double of 12," referring to the number of apostles.

The ring traditionally had been emblazoned with the large seal on it that was used by popes to seal apostolic letters. This time, the seal will be a separate piece, but the ring will have the same picture on it: the figure of St. Peter casting his net from a fisherman's boat.

Valenziano also said that the Mass would mark a clear return to early church traditions. Pope Benedict will don a style of pallium that has not been worn for about a millennium. The longer, wider cloth is embroidered with five red silk crosses, three of them pierced with pins to signify the nails used to crucify Jesus Christ. The black hem represents the hooves of sheep.

After the Mass, the pope is expected to make a tour of the square, though Valenziano suggested that he would probably not use the "popemobile," often used by Pope John Paul II.

Italian officials plan to bolster security today, closing airspace within a five-mile radius of the Vatican from 8 a.m. to 4 p.m. and shutting down one of Rome's airports.

Representatives and religious leaders of more than 30 countries were expected this morning, including Chancellor Gerhard Schroeder of Germany; Prince Albert II of Monaco; King Juan Carlos and Queen Sofia of Spain; the archbishop of Canterbury, the Most Rev. Rowan Williams; Chrisostomos, a top envoy for Ecumenical Patriarch Bartholomew I, the spiritual leader of the world's Christian Orthodox; and a senior representative of the Russian Orthodox Church, Metropolitan Kirill.

The American delegation will be led by President Bush's younger brother Jeb Bush, the governor of Florida.

The installation ceremony used to be called a coronation — when popes wore crowns to signify their political and spiritual powers. Pope Paul VI did away with the tiara, but the rite of the installation wasn't finalized when John Paul I was elected in 1978 or when John Paul II became pope about a month later, and "substitute" measures were used.

The new rituals were approved by Benedict the day after he was elected.

The installation ceremonies continue after the Mass, with visits by Benedict to the three other main basilicas of Rome: St. Paul Outside the Walls, St. John Lateran and St. Mary Major.

Benedict travels first to St. Paul Outside the Walls tomorrow — a pilgrimage because Paul is considered the co-founder of the church with Peter. Benedict will read a biblical passage from the Letter from St. Paul to the Romans to signify his links to the city.

He will further that symbolism on May 7 by presiding over a Mass at St. John Lateran, his cathedral as bishop of Rome. Finally, he will pay homage to an icon of Mary at St. Mary Major basilica before returning to the Vatican.