Pope Leo XIV set the tone for his papacy Sunday with a call to stop exploiting nature and marginalizing the poor, before an audience including JD Vance and tens of thousands of pilgrims.

Ten days after Chicago-born Robert Francis Prevost became the first US head of the world's 1.4 billion Catholics, he celebrated his inaugural mass in St Peter's Square.

The 69-year-old began the day by making his debut tour in a popemobile, standing up in the custom-made white vehicle and smiling, waving and blessing the cheering crowds at the Vatican.

In front of dignitaries including US Vice President Vance and Ukraine's leader Volodymyr Zelensky, he then gave a homily calling for the Church to be a transformational force in a world of division and hatred.

"In this our time, we still see too much discord, too many wounds caused by hatred, violence, prejudice, the fear of difference, and an economic paradigm that exploits the Earth's resources and marginalizes the poorest," he said. 

The new pontiff, who spent many years as a missionary in Peru, also warned against "closing ourselves off in our small groups".

"We are called to offer God's love to everyone, in order to achieve that unity which does not cancel out differences but values the personal history of each person and the social and religious culture of every people," he said.

Prevost, who was made a cardinal only in 2023 and is unknown to many Catholics, has repeatedly emphasized the importance of peace and social justice in his first few days as pope.

Inacia Lisboa, 71, a pilgrim from Cape Verde who lives in Rome, told AFP at the Vatican on Sunday that Leo had already "entered my heart".

She said she wanted him to "pray for us all, for peace in the world — we need it so much".

First US pope

Leo has made history as first pontiff from the United States, and his home country was represented on Sunday by Vance, who converted to Catholicism in 2019, and Secretary of State Marco Rubio, also a Catholic. 

Before becoming pope, Leo reposted on his personal X account criticism of President Donald Trump's administration over its approach to migration and also pilloried Vance, but the account is no longer accessible.

Vance was the last world leader to meet with Pope Francis, the day before the Argentine died on 21 April after 12 years as pontiff.

Leo's elevation has sparked huge enthusiasm in the United States, but also some consternation elsewhere that a country with an already outsize political and military role in the world now boasts one its foremost spiritual leaders.

"There is going to be extra weight because he is American, I think there's going to be a lot of extra eyes, and maybe criticisms," said Sophia Tripp, a 20-year-old student visiting from Leo's hometown of Chicago.

But she said she hoped he would "bring people together", adding: "We are all human, and we should just all be loving to one another."

Other guests on Sunday included German Chancellor Friedrich Merz and Italian Prime Minister Giorgia Meloni.

Peruvian President Dina Boluarte, European Commission President Ursula von der Leyen, Canadian Prime Minister Mark Carney, Israeli President Isaac Herzog, Colombia's Gustavo Petro and a host of European royals also attended.

Italian authorities deployed thousands of security officers for the event, alongside snipers on rooftops and anti-drone operations.

'Fear and trembling'

Leo XIV was elected the 267th pope on 8 May after a conclave vote of cardinals that lasted less than 24 hours.

Succeeding the charismatic but impulsive Francis, he takes over a Church still battling the fallout of the clerical child abuse scandal, and trying to adapt to the modern world.

Leo acknowledged on Sunday some trepidation in his new role.

"I was chosen, without any merit of my own, and now, with fear and trembling, I come to you as a brother who desires to be the servant of your faith and your joy," he said.

Ahead of the mass, Leo visited the tomb of Saint Peter — who in the Christian tradition was one of the Twelve Apostles of Jesus Christ, and the first pope — located under the altar of the basilica that bears his name.

He then received the pontifical emblems — the pallium, a strip of cloth worn over the chasuble, his robe and the fisherman's ring, which is forged anew for each pope and which he will wear on his finger until he dies, when it will be destroyed.

by Alice RITCHIE / Clément MELKI