As thousands of his flock gathered for his installation Mass on Tuesday, Pope Francis urged presidents, princes, sheiks and ordinary people to protect the environment, the weakest and the poorest, with a vision mapping out a clear focus of his priorities as leader of the world's 1.2 billion Catholics.

The Argentina native is the first pope from Latin American and the first named for the 13th-century friar St. Francis of Assisi, whose life's work was to care for nature, the poor and most disadvantaged. In a fitting note, he promised that a little bit of tenderness can "open up a horizon of hope."

Interrupted by applause several times during his homily, Francis also spoke of the need to protect the environment, serve one another with love and tenderness and not allow "omens of destruction," hatred, envy and pride to "defile our lives."

Francis said his role as pope will be to open his arms and protect “the poorest, the weakest, the least important, those whom Matthew lists in the final judgment on love: the hungry, the thirsty, the stranger, the naked, the sick and those in prison."

"Today amid so much darkness we need to see the light of hope and to be men and women who bring hope to others," he said. "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, it is to let a shaft of light break through the heavy clouds," he said.

Prior to the start of Mass, Francis thrilled the crowd taking a long round-about through the sun-drenched piazza and getting out of his jeep to bless a disabled man. It was a gesture from a man whose short papacy so far is becoming defined by such spontaneous forays into the crowd and concern for the disadvantaged.

The blue and white flags from Argentina fluttered above the crowd, which Italian media initially estimated could reach 1 million. The Vatican said the actual size was between 150,000-200,000. Civil protection crews closed the main streets leading to the square to traffic and set up barricades for nearly a mile (two kilometers) along the route to try to control the masses and allow official delegations through.

Before the Mass began, Francis received the fisherman's ring symbolizing the papacy and a wool stole symbolizing his role as shepherd of his 1.2-billion strong flock. He also received vows of obedience from a half-dozen cardinals — a potent symbol given his predecessor Benedict XVI is still alive.

Today amid so much darkness we need to see the light of hope and to be men and women who bring hope to others.

- Pope Francis

A cardinal intoned the rite of inauguration, saying: "The Good Shepherd charged Peter to feed his lambs and his sheep; today you succeed him as the bishop of this church."

Some 132 official delegations attended, including more than a half-dozen heads of state from Latin America, a sign of the significance of the election for the region. Francis has made clear he wants his pontificate to be focused on the poor, a message that has resonance in a poverty-stricken region that counts 40 percent of the world's Catholics.

In the VIP section was German Chancellor Angela Merkel, U.S. Vice President Joe Biden, the Argentine President Cristina Fernandez, Taiwanese President Ying-Jeou Ma, Zimbabwe President Robert Mugabe, Prince Albert of Monaco and Bahrain Prince Sheik Abdullah bin Haman bin Isa Alkhalifa, among others.

Francis directed his homily to them, saying: "I would like to ask all those who have positions of responsibility in economic, political and social life, and all men and women of goodwill: let us be protectors of creation, protectors of God's plan inscribed in nature, protectors of one another and of the environment."

His history of living with the poor and working for them while archbishop of Buenos Aires seems to have resonated with ordinary Catholics who say they are hopeful that Francis can inspire a new generation of faithful who have fallen away from the church.

"I think he'll revive the sentiments of Catholics who received the sacraments but don't go to Mass anymore, and awaken the sentiments of people who don't believe anymore in the church, for good reason," said Judith Teloni, an Argentine tourist guide who lives in Rome and attended the Mass with a friend.

"As an Argentine, he was our cardinal. It's a great joy for us," said Edoardo Fernandez Mendia, from the Argentine Pampas who was in the crowd. "I would have never imagined that it was going to be him."

Recalling another great moment in Argentine history, when soccer great Diego Maradona scored an improbable goal in the 1986 World Cup, he said: "And for the second time, the Hand of God came to Argentina."

Francis has made headlines with his simple style since the moment he appeared to the world on the balcony of St. Peter's Basilica, eschewing the ermine-lined red velvet cape his predecessor wore in favor of the simple papal white cassock, then paying his own bill at the hotel where he stayed prior to the conclave that elected him pope.

He has also surprised — and perhaps frustrated — his security detail by his impromptu forays into the crowds.

For nearly a half-hour before the Mass began, Francis toured the square in an open-air jeep, waving, shouting "Ciao!" to well-wishers and occasionally kissing babies handed up to him as if he had been doing this for years. At one point, as he neared a group of people in wheelchairs, he signaled for the jeep to stop, hopped off, and went to bless a man held up to the barricade by an aide.

"I like him because he loves the poor," said 7-year-old Pietro Loretti, who attended the Mass from Barletta in southern Italy. Another child in the crowd, 9-year-old Benedetta Vergetti from Cervetri near Rome, also skipped school to attend.

"I like him because he's sweet like my Dad."

A wax cast of the ring Francis received was first presented to Pope Paul VI, who presided over the second half of the Second Vatican Council, the 1962-65 meetings that revolutionized the church. Paul never wore it but the cast was subsequently made into the ring that Francis chose among several other more ornate ones.

Francis will receive each of the government delegations in St. Peter's Basilica after the Mass, and then hold an audience with the visiting Christian delegations on Wednesday. He has a break from activity on Thursday; a gracious nod perhaps to the fact that the Archbishop of Canterbury, Justin Welby, is being installed that day in London.

As a result, Welby wasn't representing the Anglican Communion, sending instead a lower-level delegation. All told, six sovereign rulers, 31 heads of state, three princes and 11 heads of government were attending, the Vatican said.

For Jews, Orthodox and other religious leaders, the new pope's choice of Francis as his name is also important for its reference to the Italian town of Assisi, where Pope John Paul II began conferences encouraging interfaith dialogue and closer bonds among Christians.

Based on reporting by the Associated Press.

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