Pope Benedict XVI, employing ancient symbols of power and humility, ceremonially assumed leadership of the Catholic Church on Sunday and unveiled a broad manifesto of moral and social themes he expects to pursue as head of a 1.1 billion-member congregation.
Benedict, draped in gold, presided at an outdoor Mass in St. Peter's Square, whose twin colonnades embraced hundreds of thousands of worshippers. The crowd spilled down the broad Via della Conciliazione toward the Tiber River as a variety of national flags flapped briskly in a spring breeze.
Amid all the splendor and adulation, the new pontiff began his sermon on a humble note. "At this moment, weak servant of God that I am, I must assume this enormous task, which truly exceeds all human capacity. How can I do this? How will I be able to do it?" he asked.
Then, invoking saints and Catholics at large, he said: "I am not alone. I do not have to carry alone what in truth I could never carry alone."
The massive throng, estimated at 350,000, responded with frequent if somewhat tentative applause to the new pope's words. They clapped most enthusiastically when Benedict's predecessor, Pope John Paul II, was mentioned.
Benedict smiled infrequently during the sermon, which he delivered in Italian, but looked cheerful at the end of the service. He then mounted an open, white, jeeplike automobile for a tour of the square. Well-wishers shouted "Viva!" and "Benedict, Benedict." Unlike the popemobile used by John Paul, who was the target of an assassination attempt in 1981, Benedict's vehicle was not equipped with bulletproof glass.
The Mass and sermon capped a six-day period in which Cardinal Joseph Ratzinger, the Vatican's rigorous chief defender of dogma for almost 25 years, was transformed into Pope Benedict XVI, Catholicism's 265th pontiff.
Ratzinger was well known inside the Vatican walls and at Catholic conferences. His name is a fixture in theological libraries. Since his election as pope on Tuesday, Vatican officials have worked hard to reintroduce him as Benedict, leader of all Catholics.
Sometimes it was done through official television cameras, which recorded him greeting followers on the street, blessing children and smiling expansively. Sometimes it was accomplished by way of chatty associates who described some of his personal traits -- his love of piano and Mozart, his sly humor, his willingness to do his own housekeeping.
On Sunday, he gave the widest portrait yet of his thoughts. As if in answer to the widely expressed opinion that he is dictatorial, he pledged "not to pursue my own ideas, but to listen, together with the whole church, to the word and the will of the Lord."
When he was a cardinal in charge of the Congregation for the Doctrine of the Faith, he referred to other faiths as deficient. On Sunday, he greeted "all those who have been reborn in the sacrament of baptism but are not yet in full communion with us," as well as "brothers and sisters of the Jewish people, to whom we are joined by a great shared spiritual heritage, one rooted in God's irrevocable promises.
"Finally," he concluded, "like a wave gathering force, my thoughts go out to all men and women of today, to believers and nonbelievers alike."
Along with problems of the soul, Benedict briefly addressed concrete sources of day-to-day suffering in a rare statement on social issues. "There is the desert of poverty, the desert of hunger and thirst, the desert of abandonment, of loneliness, of destroyed love. There is the desert of God's darkness, the emptiness of souls no longer aware of their dignity or the goal of human life," he said.
At one point, he sounded like an avid environmentalist.
"The Earth's treasures no longer serve to build God's garden for all to live in, but they have been made to serve the powers of exploitation and destruction," Benedict said.
He also turned his attention to world leaders. Recalling that John Paul had criticized them, Benedict said: "The pope was addressing the mighty, the powerful of this world, who feared that Christ might take away something of their power if they were to let him in. Yes, he would certainly have taken something away from them: the dominion of corruption, the manipulation of law and the freedom to do as they pleased."
The papal transition has been seamless. Long before John Paul died, Ratzinger had become a leading Vatican voice on diverse church issues. He delivered the sermon at John Paul's funeral and at a Mass celebrated just before the opening Monday of the conclave of cardinals who elected him the next day. Benedict then reappointed all the officials who served in John Paul's Cabinet. The only vacant position is his former job at the Congregation for the Doctrine of the Faith.
Yet as he emerged from St. Peter's Basilica on Sunday morning, Benedict seemed tense. His eyes shifted from side to side and his voice occasionally wavered as he addressed the audience, which included President Bush's brother, Gov. Jeb Bush of Florida, German Chancellor Gerhard Schroeder, King Juan Carlos and Queen Sofia of Spain, and other dignitaries.