POPE Francis has urged princes, world leaders and thousands of ordinary people at his installation Mass 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, echoed the gentleness for which St Francis is known saying tenderness can "open up a horizon of hope."

The Vatican estimated between 150,000-200,000 people attended the Mass, held under bright blue skies after days of chilly rain and featuring flag-waving fans from around the world.

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 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, 76, thrilled the crowd at the start of the Mass by taking a trip through the 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.

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.