Pope Francis greets the crowd in St. Peter's Square in Vatican City… (Vincenzo Pinto / AFP/Getty…)
VATICAN CITY — Amid elaborate ritual and ancient symbols of Christendom, Pope Francis began the first official day of his pontificate Tuesday by setting out a vision for the Roman Catholic Church of mutual caring and of concern for the environment, urging followers to pay special attention to society's poor and neglected.
Before tens of thousands of pilgrims and dignitaries gathered for his inauguration in St. Peter's Square, the pontiff made clear that his papacy would reflect the themes of service and love of nature so closely identified with the saint after whom he named himself, Francis of Assisi.
"Let us be protectors of creation, protectors of God's plan inscribed in nature, protectors of one another and of the environment," the pope said. "Let us not allow omens of destruction and death to accompany the advance of this world!"
PHOTOS: Pope Francis' inauguration mass
He called on government leaders, and himself, to "protect all of God's people and embrace with tender affection the whole of humanity, especially the poorest, the weakest, the least important."
The words drew loud applause across the sun-splashed piazza, which was packed with people eager to see the church's 266th pope— and the first from the Americas — take office.
It was a formal installation Mass for an informal pope, an event full of pageantry for a man who has spent his priestly life eschewing ostentation. During the 90-minute ceremony, Francis was presented with the "fisherman's ring" and a special stole, known as a pallium, denoting his role as a shepherd, symbols of the papacy that date to the 4th century.
But he also showed his continued willingness to depart from tradition in order to draw closer to ordinary folk and become, in many ways, the people's pope.
In the week since his election as pontiff by the church's cardinals, the Argentine-born Francis has endeared himself to Catholics and non-Catholics alike with his approachability, warm manner and the humble lifestyle he adopted as a Jesuit, a religious order known for its emphasis on simplicity and service.
For the inauguration, he ordered that the Mass be shortened from its usual length of more than two hours and asked that only a few cardinals come up to make their formal pledge of allegiance.
Dressed in simple white vestments, the 76-year-old pontiff delighted pilgrims by zipping through St. Peter's Square in an open-backed jeep for about half an hour before the ceremonial events began, waving and kissing babies handed up to him. At one point, he dismounted and went over to lay hands on a disabled man while Swiss Guards tried to keep the crowd behind barriers.
The square was a sea of national flags and banners, including one with the Hebrew word "shalom" and another declaring, "Buongiorno, Francesco," or "Good day, Francis," a riff on his casual greeting to the throng present Sunday for his first Angelus blessing.
"This is the pope who can fill empty churches," said Anna Pangrazi, 38. "Francis told us he was here to serve us, and that will encourage us to serve others."
Francis succeeds the retired pope emeritus, Benedict XVI, a bookish theologian who never appeared entirely comfortable among the crowds.
The new pontiff paid homage to his predecessor in his homily, but where Benedict, at his installation in 2005, preached on the need for unity and spoke grimly of an earthly wasteland haunted by alienation and suffering, Francis focused on love of neighbor and creation, and on hope.
"To protect creation, to protect every man and every woman, to look upon them with tenderness and love, is to open up a horizon of hope," he said, with majestic St. Peter's Basilica behind him. "It is to let a shaft of light break through the heavy clouds."
He cited the example of Joseph, the patron saint of the church, whose feast day fell on Tuesday.
"Let us never forget that authentic power is service and that the pope, too, when exercising power, must enter ever more fully into that service which has its radiant culmination on the cross," Francis said. "He must be inspired by the lowly, concrete and faithful service which marked St. Joseph."
He preached in Italian, but prayers were read aloud in several languages, including French and Chinese. One of the Bible readings was delivered in English, another in Spanish.
Reflecting the pope's concern for ecumenism, leaders of other religions and Christian denominations were present as well, most notably Ecumenical Patriarch Bartholomew I, the first leader of Orthodox Christians to attend a papal installation since the church split from Rome 1,000 years ago.
Dignitaries included Vice President Joe Biden, President Cristina Fernandez de Kirchner from the pope's native Argentina and crowned heads from several countries. After the Mass, the pope received the various VIPs for more than an hour.
Francis inherits a church that is growing in places such as Asia and Africa but is declining as a proportion of the population in Europe. Serious scandals have tarnished the church's image.
Most of Francis' clerical career has been spent in Argentina, where he was known as Cardinal Jorge Mario Bergoglio, archbishop of Buenos Aires.
On Tuesday, he surprised his compatriots by telephoning a priest at the cathedral in the Argentine capital and having his voice patched through to speakers in the Plaza de Mayo, where a crowd had assembled to watch video of the installation Mass. He entreated them not to "forget this bishop who is far away but who loves you very much. Pray for me!"
Special correspondent Tom Kington in Vatican City contributed to