An estimated 250,000 pilgrims and many world leaders and royals, including U.S. Vice President JD Vance, Ukrainian President Volodymyr Zelenskyy; Canadian Prime Minister Mark Carney and UK Prince Edward are expected to attend St Peter's Square for the first Pope Leo Xiv.

The service started at 10 a.m. local time and marked the official beginning of the first American Pope in the history of the Roman Catholic Church.

Chicago Cardinal Robert Prevost, 69, was elected as the late Pope Francis earlier this month, after a meeting lasted less than 26 hours.

Other world leaders participating include Israeli President Isaac Herzog, Argentina President Javier Milei and Australian Prime Minister Anthony Albanese.

Just like the tradition of the first Pope Sunday Mass, Leo is expected to reach Peter’s square, climb onto Popemobile, and spend some time saying hello to the crowd.

The Pope then enters St. Peter's Basilica, where he will receive two gifts that symbolize the Pope.

The first was the gold medal fisherman's ring, which was named in memory of St. Peter, the first pope fisherman. The ring represents the beginning and end of the pope, and when the pope died, it was destroyed by the high cardinal. Until 1842, the ring was used to seal the official documents signed by the Pope.

The second gift is the wool pallium of the Lamb, which symbolizes the Pope's role as a shepherd and is draped on his shoulders before the masses begin.

JD Vance briefly met Pope Francis in the Vatican the day before his death, and U.S. Secretary of State Marco Rubio will represent the United States.

Prior to being elected as Pope, Prevost criticized Donald Trump's administration in several positions on his X account, focusing primarily on government policies on immigration.

In February, he also shared an op-ed article published on X in the National Catholic Reporter titled “JD Vance is wrong: Jesus does not ask us to rank our love for others”.

The Vatican has not confirmed or denied that social media posts are true. Vance and Trump also clashed with Pope Francis with immigration.

Prevost's criticism has led to strong opposition from Trump's tough conservative supporter of the Pope. "He is an anti-Trump, anti-Maga, the borders of loved ones and a Marxist like Pope Francis," said activist Laura Loomer on X.

The Vatican said last week that Pope Leo XIV will remain active on social media through the official Pope accounts of X and Instagram. On a platform owned by X's platform Elon Musk, Trump's billionaire is trying to cut government spending and lay off employees, @pontifex communicates a total of 52 million followers in nine languages.

Leo wrote in his first post on X: "The Holy See is willing to help the enemy meet, so they can look at each other's eyes, so they can give people the dignity they deserve: the dignity of peace. Shake hands. I said to the leaders of the state, and said to the leaders of the state: Let us meet; let us negotiate; let us negotiate!"