Pope Leo XIV appealed to the world's major powers for "no more war" during his first Sunday message to crowds in St. Peter's Square in Rome since he was elected on 8 May.
The new pope called for an "authentic and lasting peace" in the war in Ukraine, a ceasefire in Gaza, and the release of all Israeli hostages held by militant group Hamas.
He also welcomed Saturday's ceasefire agreement between India and Pakistan and said he was praying to God to grant the world the "miracle of peace".
The Pope also recited the Regina Caeli prayer, in honour of the Virgin Mary, to the tens of thousands of people gathered in St. Peter's Square and on the Via della Conciliazione leading to the Vatican.
Speaking from the central balcony of St Peter's Basilica, he said: "I would like to address the powerful people of the world, repeating the always-current call: 'no more war'," echoing a frequent call of the late Pope Francis.
He noted the recent 80th anniversary of the end of World War Two, in which some 60 million people were killed.
Leo said today's world was living through "the dramatic scenario of a Third World War being fought piecemeal," again repeating a phrase coined by Francis.
The crowds broke into applause at his call for peace.
The Pope said he carries in his heart the "suffering of the beloved people of Ukraine".
Hours after Russian President Vladimir Putin proposed direct talks with Ukraine aimed at ending the bloody three-year war, Leo appealed for negotiations to reach an "authentic, just and lasting peace".
He also said he was "profoundly saddened by what is happening in the Gaza Strip", calling for an immediate ceasefire, the delivery of humanitarian aid and release of the remaining hostages held by Hamas.
Israel has stopped all humanitarian aid entering Gaza and resumed strikes following the collapse of a two-month ceasefire and hostage exchange agreement.
Pope Leo was chosen as the new leader of the Catholic Church on Thursday, after a two-day conclave in Vatican City.
He is the first US-born pontiff and was a relative unknown on the world stage before his election.
Formerly known as Cardinal Robert Prevost, he said he had chosen the name Leo after a 19th-century Pope known for his teachings on social justice.
He held his first Mass as Pope in the Sistine chapel on Friday. During a meeting with cardinals on Saturday he described himself as an unworthy choice for Pope and vowed to continue his predecessor's "precious legacy".
Leo cited one of Francis’s key priorities of making the Catholic Church more attentive to lay people and inclusive.
In all of his appearances since his election, the new pope has made no mention of the country of his birth, angering some US conservative commentators.
Before becoming pontiff, he was not shy about criticising President Donald Trump and Vice President JD Vance, notably their clampdown on immigration.
In February, Prevost reposted an article headlined, "JD Vance is wrong: Jesus doesn't ask us to rank our love for others."
In his first address on Thursday evening he told the crowds he wanted "to walk together with you as a united Church searching all together for peace and justice".
Leo will be formally inaugurated on 18 May at a mass in St Peter's Square.
(with newswires)
And the governments of the world go back to ignoring the pope...