VATICAN CITY (AP) – The official inauguration of Pope Leo XIV's pontificate is taking place in a grand ceremony on Sunday, showcasing a blend of ancient rituals and modern elements. This historic event, marking the first American pope, is expected to attract presidents, princes, and tens of thousands of people to St. Peter’s Square.

Pope Leo XIV, a 69-year-old Augustinian missionary originally from Chicago, will commence the day with a procession through the square in the iconic popemobile. This open-topped vehicle symbolizes the papal mission to connect with people worldwide. The popemobile was last seen on Easter Sunday with Pope Francis, and it was also utilized to transport Francis' casket across Rome last month to its final resting place.

Although Leo appears somewhat more reserved than his predecessor Francis, he is expected to engage warmly with the crowds, including those presenting babies for blessings. Following the public tour in the square, Leo will enter St. Peter’s Basilica to partake in a solemn ceremony meant to signify his commitment to leading the Catholic Church. He will begin by praying at the tomb of St. Peter, the first pope, before processing back to the square for a Mass.

The ceremony is governed by strict diplomatic protocols regarding seating arrangements, with both the United States and Peru receiving prominent spots due to Leo’s dual citizenship. U.S. Vice President JD Vance, a recent convert to Catholicism, will lead the American delegation alongside U.S. Secretary of State Marco Rubio. Peruvian President Dina Boluarte is among several heads of state present, including Ukrainian President Volodymyr Zelenskyy.

During the Mass, Leo will receive two significant papal symbols: the pallium, a lambswool stole representing a shepherd’s duty to his flock, and the fisherman's ring, which symbolizes Peter’s call from Jesus to lead. In this installation, a shift is noted from having all cardinals pledge obedience directly to the pope; now, representatives from various Church sectors will also partake in this rite.

Since his election, Leo has highlighted key priorities, emphasizing a focus on peace amidst ongoing conflicts, including those in Ukraine and Gaza. His priorities also extend to addressing the challenges posed by artificial intelligence, drawing parallels with societal challenges encountered during the Industrial Revolution, akin to concerns faced by his namesake, Pope Leo XIII.

At the conclusion of Mass, Leo will deliver a final blessing before greeting heads from over 150 official delegations in the basilica. Security measures are anticipated to be rigorous, reflecting the arrangements for Francis’ funeral, which attracted an estimated 250,000 attendees. Authorities are preparing for a similar turnout on Sunday, with large screens and sanitation facilities set up throughout the vicinity.