VATICAN CITY — Striking a tone of radical humility that has already become his trademark, Pope Francis offered a passionate pledge in his installation Mass on Tuesday to serve “the poorest, the weakest, the least important,” urging world leaders to protect human life and the environment and use tenderness to inspire hope.

Seated on a throne under a canopy in front of St. Peter’s Basilica before several hundred thousand faithful and the cardinals of his church, Francis implored dozens of visiting world leaders not to allow “omens of destruction and death to accompany the advance of this world.”

“Today, too, amid so much darkness, we need to see the light of hope and to be men and women who bring hope to others,” the pope added, speaking before heads of state from his native Argentina to Zimbabwe, including Vice President Joseph R. Biden Jr. and the German chancellor, Angela Merkel.