Vatican City. Pope Leo XIV officially begins his pontificate Sunday in a ceremony blending ancient ritual, powerful symbols, and a nod to modern celebrity, all before presidents, princes, and tens of thousands expected to pack St. Peter’s Square for the installation of history’s first American pope.

Leo will kick off the day with his first popemobile tour through the square. The open-topped vehicle, now synonymous with the papacy’s global visibility, allows popes to connect directly with the faithful at home and abroad.

It was in St. Peter’s Square that Pope Francis took his final popemobile ride on Easter Sunday. And it was atop a modified popemobile that his casket was carried across Rome last month to its final resting place.

Leo, a 69-year-old Augustinian missionary from Chicago, appears more reserved than Francis. But attention will be on how he handles the throngs of pilgrims, tourists, and curious onlookers, as well as the inevitable parade of babies passed to him for a blessing.

Following the festive tour, Leo will enter the basilica for the solemn ceremony inaugurating his ministry. The rites begin with a prayer at the tomb of St. Peter beneath the main altar, then continue outside in the square with the celebration of Mass.

Diplomatic protocol governs seating arrangements, with the United States and Peru receiving front-row seats due to Leo’s dual citizenship. US Vice President JD Vance, a Catholic convert who previously clashed with Francis over the Trump administration’s mass deportation policy, leads the American delegation alongside Secretary of State Marco Rubio. Peruvian President Dina Boluarte is among roughly a dozen heads of state attending, including Ukrainian President Volodymyr Zelenskyy.

During the Mass, Leo will receive the two most potent symbols of the papacy: the lambswool pallium and the fisherman’s ring. The pallium, draped over his shoulders, represents the shepherd carrying his flock. The ring, his official seal, evokes Christ’s call to Peter to cast his nets and become a fisher of men.

Another key moment is the rite of obedience. While once all cardinals pledged allegiance to the new pope, modern installations include representatives from various groups --cardinals, bishops, priests, deacons, nuns, married couples, and young people-- symbolizing the universality of the Church.

Since his election, Leo has outlined priorities focused on peace. It remains to be seen whether he will use his installation homily to lay out a broader mission, as some predecessors did.

In his Oct. 22, 1978 homily, St. John Paul II proclaimed: “Be not afraid! Open wide the doors to Christ!” --a line that echoed throughout his papacy. Benedict XVI quoted it in his April 25, 2005 homily and reflected on the unity symbolized by the pallium and ring. Francis, in his March 19, 2013 homily, emphasized environmental stewardship, a theme that defined much of his pontificate.

Leo has pledged efforts toward peaceful resolutions in Ukraine, Gaza, and elsewhere. He also identified artificial intelligence as a pressing global challenge, drawing parallels to the human dignity issues raised during the Industrial Revolution, a key concern of his namesake, Pope Leo XIII (1878–1903).

After the homily and final blessing, Leo will return inside the basilica to greet leaders of the more than 150 official delegations attending.

Security will be tight, as it was for Francis’ April 26 funeral, which drew an estimated 250,000 people. Authorities expect a similar turnout Sunday. Giant screens have been installed in the square, nearby boulevards, and adjacent piazzas, along with dozens of portable toilets in a nearby park.