Security forces went on alert around the Vatican (search) on Saturday and cleared streets for an expected 500,000 pilgrims arriving for the ceremony to formally install Pope Benedict XVI (search) and offer the pontiff a major chance to set the tone for his papacy.

The open-air Mass in St. Peter's Square (search) planned for Sunday also gives other religious leaders an opportunity to closely assess any new directions in the Vatican.

Christian envoys expected at the ceremony cover a broad range: Orthodox clerics, Anglicans, mainstream Protestants and evangelical delegates such as members of the Rev. Billy Graham's association. The list suggests Christian congregations are cautiously extending a hand to Benedict despite lingering suspicions over his interest in closer ties.

In 2000, while serving as the Vatican's chief overseer for doctrine, he issued "Dominus Iesus," a document that upset non-Catholics by framing salvation from only a Catholic perspective. But Wednesday — a day after his election as pope — Benedict promised to seek greater ties with all Christians and open "sincere dialogue" with other faiths.

But few top Islamic leaders are expected at the Mass. Jewish presence could be complicated by the weeklong Passover holiday, which began Saturday.

Political and ceremonial dignitaries planning to attend include German Chancellor Gerhard Schroeder, Prince Albert II of Monaco and Florida Gov. Jeb Bush (search), who is leading the U.S. delegation that includes 21 congressmen. Rome's second airport, Ciampino, was closed to commercial flights Saturday to allow official delegations to arrive.

A no-fly zone also will be imposed within a five-mile radius of the Vatican for most of Sunday. In a repetition of the security during John Paul II's funeral April 8, Italian forces will have anti-missile systems in place and warplanes on patrol alongside NATO surveillance aircraft.

The inaugural Mass will start at 10 a.m. (4 a.m. EDT) Sunday and last about two hours.

For Benedict, the Mass also will bring him back to the steps of St. Peter's Basilica, where he gave a moving funeral homily for the late pope that could have helped sway fellow cardinals in his favor when it came to select the new leader of the 1.1-billion member church.

The pope's funeral brought a sea of Polish flags. Benedict's Mass — known as the Ceremony of Investiture — should be strongly flavored by an estimated 100,000 German pilgrims honoring the first pope from their region in 1,000 years. Italian authorities predict as many as 500,000 people filling St. Peter's Square and surrounding streets.

The 78-year-old pontiff will have his biggest stage for messages or gestures to point the way for his papacy. Already, his relaxed manner and promises of openness have softened the rigid reputation he earned during 24 years as the Vatican's guardian of doctrine.

He plans to greet pilgrims after the Mass in a new style of motorized transport, said Crispino Valenziano, the Vatican's deputy master of ceremonies.

"It's not the `popemobile' as we've been used to," he said, declining to give details. "The pope will certainly go around the square, but not on foot."

In 1978, John Paul walked to the crowds after his installation. But security worries — and memories of the 1981 assassination attempt on his life — have placed limits on papal movements.

At the ceremony, the pope will be joined by his brother, the Rev. Georg Ratzinger, who received applause at Rome's Leonardo Da Vinci airport after arriving Saturday.

The religious leaders due to attend include Archbishop of Canterbury Rowan Williams; Metropolitan 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.

During the Mass, Benedict will receive his papal Fisherman's Ring as well as the pallium — a narrow stole of white wool embroidered with five red silk crosses — which symbolizes his pastoral authority.

On Saturday, the pope's belongings were moved from his former residence just outside the Vatican walls to the papal residence overlooking St. Peter's Square. Workers lugged boxes labeled "Handle With Care."

Earlier, Benedict addressed journalists to remind them of the ethical dimensions of their profession and thanking them for their coverage of the "historically important" events during the papal transition.

He also promised to maintain the "open and sincere" dialogue of his predecessor, but he did not take questions or meet individual reporters. In contrast, John Paul fielded questions for 40 minutes after reading a prepared speech in his first meeting with journalists in 1978.

Benedict emphasized the need for "clear references of the ethical responsibilities" of the media, stressing the need for a "sincere search for the truth and the safeguarding of the centrality and the dignity of the person."

He read portions of his speech in Italian, English, French and his native German, joking that "since we are in Rome" he had to return to Italian. Before his remarks, the pope chuckled during a nearly one-minute ovation from the hundreds of pilgrims also at the session.

Benedict's immediate travel plans are unclear. He expects to attend World Youth Day in Cologne, Germany, in August. Polish officials have invited Benedict to visit their country.

The Polish news agency PAP, citing an anonymous source, reported that Benedict will travel to Poland this year after the country's general elections scheduled by Sept. 25.