VATICAN CITY (AP) - Pope Francis urged princes, presidents, sheiks and thousands of ordinary people gathered for his installation Mass on Tuesday to protect the environment, the weakest and the poorest, 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."

Francis was interrupted by applause several times during his homily, including when he 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 the role of the pope is to open his arms and protect all of humanity, but "especially 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.

Francis thrilled the crowd at the start of the Mass by 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 kilometres) 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.

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.

But it is Francis' history of living with the poor and working for them while archbishop of Buenos Aires that 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.