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Pope Leo XIV celebrates inaugural mass after touring St. Peter's Square in popemobile

Pope Leo XIV, formerly Cardinal Robert Prevost, was elected pontiff on May 8.

An open-topped popemobile carrying Pope Leo XIV drove on Sunday morning through St Peter's Square at the Vatican, where thousands gathered ahead of the newly elected American pontiff's inaugural mass.

The mass, called the Holy Mass for the Beginning of the Pontificate, began in St. Peter’s Basilica in Vatican City at 10 a.m. local time, the Vatican Press Office said.

Pope Leo XIV greets the crowd from the popemobile before a Holy mass for the beginning of his pontificate, in St Peter's square in The Vatican on May 18, 2025.

Tiziana Fabi/AFP via Getty Images

MORE: Pope Leo XIV to celebrate inaugural mass Sunday, thousands expected to gather

Attendees at the mass in St. Peter’s Basilica, which marks the official beginning of Leo’s papacy, included Vice President JD Vance and Secretary of State Marco Rubio, along with other religious and political figures.

PHOTO: Ukraine's President Volodymr Zelenskyy, left, and Vice President JD Vance greet each other as Secretary of State Marco Rubio, center, looksin St. Peter's Square attended by heads of state, royalty and ordinary faithful, Sunday, May 18, 2025.

Ukraine's President Volodymr Zelenskyy, left, and Vice President JD Vance greet each other as Secretary of State Marco Rubio, center, looks on ahead of Pop...Show more

Gregorio Borgia/AP

Vance and Ukrainian President Volodymyr Zelenskyy greeted each other and shook hands prior to the mass.

Leo, who was formerly Cardinal Robert Prevost, is a 69-year-old native of Chicago, Illinois. Elected on May 8, he's the first American to lead the church.

Pope Leo XIV on his popemobile tours St. Peter's Square at the Vatican prior to the inaugural Mass of his pontificate, Sunday, May 18, 2025.

Andrew Medichini/AP

Pope Francis, his predecessor, died on April 21, after he was discharged from a weeks-long hospital stay.

This is a developing story. Please check back for updates.

ABC News' Megan Forrester contributed to this report.