Pope Leo XIV rode into St. Peter's Square on a popemobile for the first time on Sunday as tens of thousands of people, including dozens of world leaders, celebrated him as the new leader of the world's 1.4 billion Catholics.

Crowds crammed the square and surrounding streets in Rome in the biggest gathering at the Vatican since the funeral of Leo's predecessor, Pope Francis, on April 26.

Well-wishers carried U.S. and Peruvian flags to cheer the first pope from the United States, who will celebrate his inaugural Mass later Sunday morning.

Born in Chicago, the 69-year-old pontiff spent many years as a missionary in Peru and also has Peruvian citizenship, meaning he is also the first pontiff tied to that South American nation.

One person in the crowd shouted out "White Sox, White Sox," referring to the Chicago baseball team at the bottom of the American League standings. Leo has never abandoned the team despite its recent lacklustre performance and his fellow diehard fans hope his support can lift them out of the doldrums.

People chanted "Viva il Papa" (Long Live the Pope) and "Papa Leone", his name in Italian.

The open-topped popemoble was surrounded by more than a dozen security guards as it drove through the square and down the long boulevard that leads to the Tiber River, for the Leo's first ride.

The vehicle moved quickly, but briefly stopped twice so Leo could bless three babies.

Robert Prevost, a relative unknown on the world stage who only became a cardinal two years ago, was elected pope on May 8 after a short conclave of cardinals that lasted barely 24 hours.

Francis, an Argentine, died on April 21 after leading the Church for 12 often turbulent years during which he battled with traditionalists and championed the poor and marginalised.

U.S. Vice President JD Vance, a Catholic convert who clashed with Francis over the Trump administration's hard-line immigration policies, is leading a U.S. delegation alongside Secretary of State Marco Rubio, who is also Catholic.

Ukrainian President Volodymyr Zelenskiy also attended, as he did at Francis' funeral when he had face-to-face talks with U.S. President Donald Trump in St. Peter's Basilica.

Zelenskiy last met Vance in February in the White House, when the two men clashed fiercely in front of the world's media.

Also attending the Vatican ceremony were the presidents of Peru, Israel and Nigeria, the prime ministers of Italy, Canada and Australia, German Chancellor Friedrich Merz and European Commission President Ursula von der Leyen.

Many European royals took their place in the VIP seats near the main altar, including Spanish King Felipe and Queen Letizia.

FOCUS ON PEACE

In various sermons and comments since his election as pope, Leo has praised Francis repeatedly but has not offered many hints about whether he will continue with the late pontiff's vision of opening the Church up to the modern world.

His homily on Sunday is likely to indicate some of the priorities for his papacy, having already made clear over the past 10 days that he will push for peace whenever possible.

His first words in an appearance to crowds in St. Peter's Square on the night of his election were "Peace be with you all," echoing words Catholics use in their celebrations.

In a May 14 address to officials of the eastern Catholic Churches, many of whom are based in global hot spots such as Ukraine and the Middle East, the new pope pledged he would make "every effort" for peace.

He also offered the Vatican as a mediator in global conflicts, saying war was "never inevitable".

Sunday's Mass will feature prayers in several languages, in a nod to the global reach of the 1.4-billion member Church, including Latin, Italian, Greek, Portuguese, French, Arabic, Polish, and Chinese.

As part of the ceremony, Leo will also formally receive two items as he takes up the papacy: a liturgical vestment known as a pallium, a strip of lambswool which represents his role as a shepherd, and a special band known as the fisherman's ring.

The ceremonial gold signet ring is specially cast for each new pope and can be used by Leo to seal documents. It features a design of St. Peter holding the keys to Heaven and will be broken after his death, marking an end to his papacy.

(Reuters)

With first ride in a popemobile, Leo XIV starts big inaugural ceremony before tens of thousands