— The Catholic Church inaugurated Pope Francis as its first Latin American and Jesuit head Tuesday morning, vesting the Argentine with the trappings of authority at an open-air Mass in St. Peter’s Square attended by more than 150,000 people.

After waving and smiling from the open back of a popemobile that rolled through the crowd, Francis, dressed in a white cassock and white skullcap, descended to the tomb of the apostle Peter, inside St. Peter’s Basilica.

Flanked by cardinals, the 266th pope knelt in prayer, then rose to offer incense. As church choirs chanted, Francis emerged from the basilica carrying a gold cross. He proceeded down a wide nave and behind a double-file line of the church’s cardinals, who were clad in golden robes and cream-colored miters.

As the new pope stepped into the sunshine, Argentines in the front row of the crowd waved their blue-and-white flags.

Representatives from 132 nations, including many heads of state, sat opposite 250 red-capped cardinals and purple-­clad bishops who had taken their seats atop the basilica stairs. Leaders of Christian, Jewish, Muslim, Buddhist and Jain faiths sat together in a section just below the steps.

In the crowd, one banner read “Buon giorno Francis,” a nod to the pope’s casual style and his initial greeting of “buona sera” from the basilica’s balcony on the night of his election Wednesday. Another banner read, “Go Francis and Fix the Church,” reflecting the hope that the Argentine would prove to be a reformer in substance as well as style.

Perhaps the most notable sign of the optimism accompanying the beginning of Francis’s pontificate was the presence of Bartholomew I, the Istanbul-based leader of the world’s Orthodox Christians. He was the first Orthodox Christian patriarch to attend a papal inauguration since the great schism between Orthodoxy and Catholicism nearly a millennium ago.

Under a crimson canopy on the basilica steps, Cardinal Jean-Louis Pierre Tauran — who announced Francis to the world with a call of “Habemus papam” on the balcony of St. Peter’s last week — draped the pontiff’s shoulders with the papal pallium. The woolen scarf recalls the sheep that Jesus carried on his shoulders and symbolizes the pope’s status as a good shepherd, responsible for a flock of 1.2 billion Catholics.

Cardinal Angelo Sodano, dean of the College of Cardinals, then fitted the pope’s right hand with the golden Ring of the Fisherman, which contains Francis’s seal and reflects the story of Christ telling the fisherman Peter that he would now be a fisher of souls. Six cardinals, representing the different orders of the College of Cardinals, then pledged their obedience to the pope.

In his homily, Francis noted the “significant coincidence” that his inauguration fell on the feast of Saint Joseph, the husband of the Virgin Mary, and the name-day of “venerable predecessor” Joseph Ratzinger, now known as Pope Emeritus Benedict XVI. Benedict last month became the first pope to resign in 600 years.

As rich as the ceremony was with the ritual and regalia of the church, it also revealed the inclination toward simplicity that Francis has demonstrated. A Greek deacon from the Eastern church chanted from the Book of Matthew, but there was no Latin complement, as is traditional at special Masses. (“There’s already a lot of Latin,” the Rev. Federico Lombardi, a Vatican spokesman, explained Monday.)

Francis’s homily highlighted the shift from Benedict’s shy, almost exclusive style, as the pope welcomed in inclusive terms not only his brother cardinals and Catholics, but also “women religious and all the lay faithful.” He also welcomed “the representatives of the other churches and ecclesiastical communities, as well as the representatives of the Jewish community and the other religious communities.”

Francis returned to Joseph as the theme of his homily, describing the saint’s role as the protector of the church’s savior and matriarch.

“How does Joseph exercise his role as protector?” the pope asked. “Discreetly, humbly and silently, but with an unfailing presence and utter fidelity, even when he finds it hard to understand.”

Not insignificantly for a church that has been criticized as overly self-referential and increasingly out of touch with the world it aims to serve, Francis added that Joseph “can look at things realistically, he is in touch with his surroundings, he can make truly wise decisions.”

Francis also struck a broadly ecumenical tone.

A pope, he said, “must be inspired by the lowly, concrete and faithful service which marked Saint Joseph, and, like him, he 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.”

Francis addressed the leaders of the nations arrayed around him, including Vice President Biden. “Please, I would like to ask all those who have positions of responsibility in economic, political and social life, and all men and women of goodwill: Let us be ‘protectors’ of creation, protectors of God’s plan inscribed in nature, protectors of one another and of the environment,” he said. “Let us not allow omens of destruction and death to accompany the advance of this world! But to be ‘protectors’, we also have to keep watch over ourselves!

With Zimbabwean President Robert Mugabe, who has been internationally sanctioned for human rights violations and political repression, seated among the dignitaries to his left, Francis said: “Let us not forget that hatred, envy and pride defile our lives. Being protectors, then, also means keeping watch over our emotions.”

The new pope spoke with a ringing optimism, seemingly eager to usher in an era of hope amid financial crises, continued wars and what the church sees as rampant global secularism and commercialism. It has also been a challenging period for the papacy, which has been beset by infighting in the Vatican, continued fallout from sex abuse by priests and cover-ups by bishops, and failure to address the decline of the church’s ranks in its former strongholds.

“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,” Francis said, calling for a “shaft of light” to break through the heavy clouds. “To protect each person, especially the poorest, to protect ourselves: This is a service that the Bishop of Rome is called to carry out, yet one to which all of us are called, so that the star of hope will shine brightly.”

His homily completed, the pope celebrated Mass, speaking softly in Argentine-­accented Latin. Cardinal Antonios Naguib, the Coptic Catholic patriarch emeritus of Alexandria, Egypt, read a prayer at the altar, and priests holding white and yellow parasols administered communion to the crowds.

Francis sat, his head often bowed, occasionally looking up and over the top of his wire-framed glasses. As choirs sang, he rose, meditated at a statue of the Virgin Mary and the infant Jesus and waved to the crowd. To the pealing of bells, he returned to the basilica and began greeting a long line of dignitaries, including presidents and royals.

“He doesn’t have the mark of a professor; he talks as a friend,” Italian Archbishop Giancarlo Maria Bregantini said. “He says big things in a simple tone.”

A feeling of jubilation lingered in the piazza outside.

“Pride,” said Felix Bialet, a 21-year-old business student from Argentina. “The head of the church is from our country.” He said he hoped that Francis would bring a new perspective to Rome that would help “modernize the church.”

“I’m curious what he is going to do,” said Dylan Hull-Nye, 21, a Catholic from Oakland, Calif., who is studying philosophy and theology at Oxford University. The pope’s humble mien and willingness to break out of the papal bubble, Hull-Nye said, left him half expecting “to run into him” somewhere inside Vatican City.

Giuseppe Russo, from Naples, expressed hope that Francis would address the corruption and petty infighting that in recent years have eroded the church’s authority. “I hope he puts them to work,” Russo said, gesturing toward the Vatican. “It’s time a column of the church fell.”