VATICAN CITY (AP) -- Pope Francis has urged princes, presidents, sheikhs and thousands of ordinary people gathered for his installation Mass to protect God's creation, the weakest and the poorest of the world.

He is officially beginning his ministry as the 266th pope with a clear focus of his priorities.

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

Francis said the pope "must open his arms to protect all of God's people and embrace with tender affection the whole of humanity, especially the poorest, the weakest, the least important."

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

Later a half-dozen cardinals approached the Argentine-born pope to vow their obedience.

The installation occurred in sun-drenched St. Peter's Square before tens of thousands of people, princes, sheiks, rabbis and presidents.

The blue and white flags from Pope Francis' native Argentina are fluttering above the crowd that Italian media estimate could reach 1 million. Emergency crews set up barricades for nearly a mile (two kilometers) along the main boulevard leading to the square to control the masses.

The installation Mass is simpler than the 2005 ceremony for Pope Benedict XVI in keeping with Francis' style, but still grand enough to draw 132 official delegations and religious leaders from around the world, including the spiritual leader of the world's Orthodox Christians who is attending for the first time since the two branches of Christianity split 1,000 years ago.