Pope Leo XIV has pledged to safeguard the Catholic Church's teachings while urging it to face modern challenges head-on, and criticised economic systems that exploit nature and the poor.
Speaking to thousands of people in St Peter's Square, the Pope 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."
"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 marginalises the poorest," the Pope said during his inauguration mass.
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".
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.
Leo acknowledged 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.
In his Regina Coeli prayer at the end of the mass, Pope Leo also noted that "in Gaza, children, families and elderly survivors are reduced to hunger".
And he recalled how "in Myanmar, new hostilities have destroyed innocent young lives".
The Pope held a private meeting with Ukrainian President Volodymyr Zelensky this afternoon.
Mr Zelensky was among hundreds of world leaders who attended the mass this morning.
"The martyred Ukraine is waiting for negotiations for a just and lasting peace to finally happen," the pontiff said ahead of the meeting.
Before the mass began, Pope Leo visited the tomb of Saint Peter 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.
Referring to St Peter, the 1st century Christian apostle from whom popes derive their authority, Leo said: "Peter must shepherd the flock without ever yielding to the temptation to be an autocrat, lording it over those entrusted to him. On the contrary, he is called to serve the faith of his brothers and sisters, and to walk alongside them."
In Pictures: Pope Leo XIV's inauguration mass
Underscoring the unbroken lineage of popes from St Peter, the choir chanted the traditional hymn "Tu es Petrus", Latin for "You are Peter".
Shortly before the mass began, the Pope made his first tour of St Peter's Square in his popemobile, smiling and waving to the crowds.
The Catholic Church's first US-born pontiff stood in the white vehicle as it drove among tens of thousands of flag-waving and cheering people at the Vatican.
Pope Leo XIV tours St Peter's Square for first time in his popemobile ahead of his inauguration mass pic.twitter.com/PyYUQ2u0u0
— RTÉ News (@rtenews) May 18, 2025
Crowds chanted "Viva il Papa" (Long Live the Pope) and "Papa Leone", his name in Italian, as his distinctive, open-topped popemobile weaved around St Peter's Square.
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 pope from that South American nation.
Robert Prevost, a relative unknown on the world stage who only became a cardinal two years ago, was elected Pope on 8 May after a short conclave of cardinals that lasted barely 24 hours.
President Michael D Higgins and his wife, Sabina, led the Irish delegation at the event.
Watch: President Michael D Higgins and his wife, Sabina, meet Pope Leo XIV
US Vice President JD Vance, a Catholic convert who clashed with Pope Francis over the Trump administration's immigration policies, led a US delegation alongside Secretary of State Marco Rubio, who is also Catholic.
Also at 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 were also in the VIP seats near the main altar, including Spanish King Felipe and Queen Letizia.
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".
In an address on 14 May 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 pontiff pledged he would make "every effort" for peace.
He also offered the Vatican as a mediator in global conflicts, saying war was "never inevitable".
