Pope Leo XIV takes his first popemobile ride through St. Peter's Square ahead of installation

Pope Leo XIV took his first popemobile ride through St. Peter’s Square on Sunday, greeting the tens of thousands of people who joined presidents, patriarchs and princes for the formal installation ceremony of history's first American pope that blends ancient ritual, evocative symbols and a nod to modern-day celebrity.

The bells of St. Peter’s Basilica tolled as Leo waved from the back of the open-topped truck that looped slowly through the square and then up and down the boulevard to it. A somewhat subdued crowd cheered “Viva il Papa” and waved plenty of Peruvian, American and Holy See flags — representing Leo's nationalities.

The popemobile tour has become synonymous with the papacy’s global reach and mediatic draw, used at home and abroad to bring popes close to their flock. It was here that Francis took his last popemobile ride on Easter Sunday, and it was on the back of a popemobile that Francis’ casket was brought across Rome last month to its final resting place.

U.S. Vice President JD Vance, one of the last foreign officials to see Pope Francis before he died, paid his respects at the Argentine pope's tomb upon arriving in Rome late Saturday and headed the U.S. delegation honoring the Chicago-born Leo.

After the public tour in the square, Leo went into the basilica to begin the solemn ceremony to inaugurate his ministry in a series of rites that emphasize the service that he's called to perform in leading the Catholic Church. The a 69-year-old Augustinian missionary prays first at the tomb of St. Peter, considered to be the first pope, under the basilica’s main altar and then processes out into the piazza for the Mass.

Strict diplomatic protocol dictates the seating arrangements, with both the United States and Peru getting front-row seats thanks to Leo’s dual citizenship. Vance, a Catholic convert who tangled with Francis over the Trump administration’s mass migrant deportation plans, is being joined by U.S. Secretary of State Marco Rubio, who arrived in Rome ahead of time to try to advance Russia-Ukraine peace talks.

Peruvian President Dina Boluarte is one of around a dozen heads of state attending, as well as Ukrainian President Volodymyr Zelenskyy. Russia is being represented by the culture minister, Olga Liubimova.

Diplomatic protocol also dictated the dress code: While most wore black, the handful of Catholic queens and princesses — Charlene of Monaco and Letizia of Spain among others — wore white in a special privilege allowed them. Three dozen of the world’s other Christian churches sent their own delegations, headed by patriarchs, reverends, ministers and metropolitans, while the Jewish community had a 13-member delegation, half of them rabbis.

U.S. seminarian Ethan Menning, 21, from Omaha, Nebraska, wrapped himself in an American flag, purchased at a truck stop in Iowa, to celebrate.

“Rome always felt like home for a Catholic, but now coming here and seeing one of our own on the throne of Peter ... it almost makes Jesus himself more accessible,” he said.

Kalen Hill, a pilgrim from the U.S., got to St. Peter's soon after the gates opened Sunday morning and said he never expected an American would lead the 1.4-billion strong church.

“I would say all the Americans are emotional about it," he said. "It is really powerful for American Catholics who sometimes feel separated from the world church to be brought in and included in this community through Pope Leo.”

During the Mass, Leo will receive the two potent symbols of the papacy: the lambswool stole, known as a pallium, and the fisherman’s ring. The pallium, draped across his shoulders, symbolizes the pastor carrying his flock as the pope carries the faithful. The ring, which becomes Leo’s official seal, harks back to Jesus’ call to the apostle Peter to cast his fishing nets.

The other symbolically important moment of the Mass is the representational rite of obedience to Leo: Whereas in the past all cardinals would vow obedience to the new pope, more recent papal installations involve representatives of cardinals, bishops, priests, deacons, nuns, married couples and young people participating in the rite.

Another change from the past is that Sunday's Mass isn't a coronation ceremony, which used to involve the pope receiving a tiara, but is merely known as a “Eucharistic Celebration for the start of the Petrine ministry of the Bishop of Rome.”

In the days since his historic election, Leo has already sketched out some of his key priorities as pope. In his first foreign policy address, he said the Holy See's three pillars of diplomacy were peace, justice and truth. In his first major economics address, he emphasized the Catholic Church's social doctrine and the search for truth. It's not known if he'll use his installation homily as a mission statement as some of his predecessors did.

After the homily and at the end of the Mass, Leo will offer a final blessing and then go into the basilica to greet the heads of the more than 150 official delegations attending.

Security was tight, as it was for Francis’ funeral on April 26, which drew an estimated 250,000 people. Rome authorities are planning for another 250,000 on Sunday. The piazza and main boulevard leading to it, and two nearby piazzas were set up with giant television screens, and dozens of portable toilets have been erected in a nearby park.

Gregory and Susan Hudak, who lived for 40 years in the Chicago area, found themselves in Rome after booking a trip in February, with just a faint hope of perhaps glimpsing the pope.

Seeing the popemobile pass by in front of them was even better than watching Michael Jordan play, said Gary Hudak, a former altar boy wearing a Chicago Bears hat.

“Originally, the only hope I had coming here was to see the inside of the Sistine Chapel," he said. "I was glad the conclave was over, because I could get to see the inside of the Sistine Chapel. Seeing the pope was not scheduled, it was a long shot hope. And this was a treasure, simple as that.”

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Associated Press writer Giovanna dell'Orto, David Biller and AP visual journalist Isaia Monteleone contributed to this report.

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