Rome, 18 May 2026 (dpa/MIA) - The celebratory Mass to inaugurate Pope Leo XIV began in the Vatican on Sunday, with hundreds of thousands of worshippers gathered in St Peter's Square to observe the historic moment.

It marks the official beginning of Leo's papacy, who was elected the 267th leader of the Roman Catholic Church on May 8. He is also the first US-born pope.

The new pontiff prayed at St Peter's tomb in St Peter's Basilica before walking out onto St Peter's Square.

During the service, the pope will receive two regalia representing the papal power: the gold signet Fisherman's Ring, which features an engraving of the Apostle Peter fishing, symbolizing Jesus' words that he would make Peter a "fisher of men," as well as a lamb's wool pallium to symbolize the pope's role as shepherd.

Dozens of state leaders, including US Vice President JD Vance - the last top politician to meet with pope Francis before he died on April 21 - royals and senior church representatives travelled to Rome to attend the inauguration.

Leo is expected to issue a personal address to worshippers during the service. During his inauguration in 2013, Francis asked Catholics to "Be protectors of God's gifts!"