VATICAN CITY, May 18, 2025 (BSS/AFP) - Tens of thousands of Catholics and politicians including US Vice President JD Vance will gather at the Vatican on Sunday for the inauguration mass of Leo XIV, the first US pope.

Ten days after the soft-spoken Robert Francis Prevost was chosen from among his fellow cardinals to lead the world's 1.4 billion Catholics, his papacy will formally begin in a ceremony at St Peter's Square.

The mass begins at 10:00 am (0800 GMT) but Pope Leo will at 9:00 am make his first foray into the crowd in the popemobile, the specially adapted vehicle often used by the late Pope Francis to motor among his flock.

Leo will then begin a ceremony rich in rites and symbols, including receiving his special papal ring, before giving a homily that will set the tone for his papacy.

After spending two decades as missionary in Peru, the 69-year-old from Chicago is unknown to many Catholics, but during the past week he has offered glimpses of the kind of leader he will be.

In meetings with journalists, clergy and diplomats, he has repeated called for peace in a world full of conflicts, and defended social justice.

In a lighter moment, the sports fan and amateur tennis player also met men's world's number one Jannik Sinner at the Vatican.

- Zelensky, Merz -

Leo's elevation has sparked huge enthusiasm in the United States, which is being represented on Sunday by JD Vance, who converted to Catholicism in 2019, and Secretary of State Marco Rubio, also a Catholic.

Before becoming pope, the new pontiff on his personal X account criticised President Donald Trump's administration over its approach to migration, and also pilloried Vance, but those messages have since been deleted.

Vance was the last world leader to meet with Pope Francis, the day before the Argentine died on April 21 after 12 years as pontiff.

Other notable guests expected include Ukraine's Volodymyr Zelensky -- who memorably met Trump in St Peter's Basilica at Francis's funeral -- and German Chancellor Friedrich Merz.

Argentine President Javier Milei, European Commission President Ursula von der Leyen, Canadian Prime Minister Mark Carney, Israeli President Isaac Herzog, Colombia's Gustavo Petro and a host of European royals are also due to attend.

Italian authorities have deployed thousands of security officers for the event, alongside snipers on rooftops and anti-drone operations.

- Fisherman's ring -

Leo XIV was elected the 267th pope on May 8 after a secret conclave vote of cardinals that lasted less than 24 hours.

Succeeding the charismatic but impulsive Pope Francis, he takes over a Church still battling the fall-out of the clerical child abuse scandal, and trying to adapt to the modern world.

Modernity is not the concern on Sunday, however. 

Although no pope has been crowned during an inauguration mass since Paul VI in 1963, the event is still a grand affair steeped in tradition.

Leo will begin by visiting the tomb of Saint Peter -- who in the Christian tradition was one of the Twelve Apostles of Jesus Christ, and the first pope -- which is located under the altar of the basilica that bears his name.

Leo will then receive the pontifical emblems -- the pallium, a strip of cloth worn over the chasuble, his robe, and the fisherman's ring, which is forged anew for each pope and which he will wear on his finger until he dies, when it will be destroyed.

With other cardinals and clergy, the pope will walk in procession into St Peter's Square, where large screens will display the proceedings to the gathered crowds.

At the end of the ceremony, the pope will greet the delegations of heads of state -- although it is not clear yet if any of them will also be accorded a one-to-one private audience.