VATICAN CITY—The Roman Catholic Church's leaders elected their new pope Wednesday in a day that was rich in ritual but broke with tradition—placing the world's 1.2 billion Catholics under the direction of a Jesuit from the New World, both firsts in Christianity's 2,000-year history.

Argentina's Cardinal Jorge Mario Bergoglio, the 76-year-old archbishop of Buenos Aires, emerged on the balcony of St. Peter's Basilica here Wednesday evening and was introduced as Pope Francis. A crowd of more than 100,000 pilgrims in the square below sounded cheers of "Viva il Papa" as he urged a "great fraternity" for the global church.

His election places the church under the leadership of a man who is known less as an educator or intellectual, as was his predecessor, than as a humble and mild-mannered shepherd of his flock. The choice of a man who has shunned the lofty trappings of a cardinal—and has spent relatively little time in the corridors of the Vatican—is likely to have broad resonance at a time when Catholicism has been losing credibility and followers in many parts of the world.

The selection of the first pontiff from the Americas is the second groundbreaking event for the Catholic Church in a month. Pope Francis's predecessor, Benedict XVI, stepped down on Feb. 28—the first pontiff in more than six centuries to resign from a position traditionally held until death.

Pope Francis immediately embraced his mission as shepherd in Rome and the world. Appearing with outstretched arms on St. Peter's main balcony, he led the pilgrims below in the Lord's Prayer and then asked them for a favor: a moment of prayer for him. He also joked with the crowd about how far he had come to be their pope.