Speaking to thousands in St Peter's Square in Rome on Sunday, Leo emphasised the need for the 1.4-billion-member church to remain rooted in its tradition without becoming insular, saying there was no room for religious propaganda or power plays in the institution's future.
Echoing the priorities of his predecessor, Pope Francis, Leo criticised the global economic system, which he said "exploits the earth's resources and marginalises the poorest".
He also warned against the centralisation of power within the papacy, saying he would seek to govern "without ever yielding to the temptation to be an autocrat".
Pope Leo XIV arrived in St Peter's Square in a popemobile for the first time 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 Pope Francis on April 26.
Well-wishers carried US and Peruvian flags to cheer the first Pope from the United States, as he celebrated his inaugural mass.
Born in Chicago, the 69-year-old pontiff spent many years as a missionary in Peru and has Peruvian citizenship, meaning he is also the first Pope from that South American nation.
People chanted "Viva il Papa" (Long Live the Pope) and "Papa Leone", his name in Italian.
The open-topped popemobile 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 on 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.
He replaces Pope Francis from Argentina, who 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.
US Vice President JD Vance, a Catholic convert who clashed with Francis over the Trump administration's hard-line immigration policies, is leading a US delegation alongside Secretary of State Marco Rubio, who is also Catholic.
Ukrainian President Volodymyr Zelenskiy is also attending and would be happy to meet other leaders, a top aide has said, as he did at Francis' funeral when he had face-to-face talks with US 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.
The presidents of Peru, Israel and Nigeria, Australian Prime Minister Anthony Albanese, the prime ministers of Italy and Canada, German Chancellor Friedrich Merz and European Commission President Ursula von der Leyen are also in Rome for the ceremony.
Many European royals were in the VIP seats near the main altar, including Spanish King Felipe and Queen Letizia.
The mass features prayers in several languages, including Latin, Italian, Greek, Portuguese, French, Arabic, Polish, and Chinese.
Leo will formally receive two items as he takes up the papacy: a liturgical vestment known as a pallium, a strip of lambswool representing 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.Â
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.
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