Pope Leo XIV appealed for no more war in a message to world powers during his first Sunday blessing at the Vatican.
He called for a “lasting peace” in the Ukraine war, a ceasefire in Gaza, as well as welcoming Saturday’s agreement between India and Pakistan.
He also recited the Regina Caeli prayer, in honour of the Virgin Mary, in his address to the crowd in St Peter’s Square.
On Saturday, he visited a shrine outside Rome and then prayed before the tomb of his late predecessor Francis inside the basilica of Santa Maria Maggiore.
Pope Leo will be formally inaugurated at a mass in St Peter’s Square next week on 18 May.
The Pope was chosen as the new leader of the Catholic Church on Thursday, following a two-day conclave in Vatican City.
It has been a busy week for the pontiff, who held his first Mass as Pope in the Sistine chapel on Friday before speaking to cardinals on Saturday.
During this meeting, he described himself as an unworthy choice for Pope, and vowed to continue the “precious legacy” of his predecessor.
He highlighted the importance of missionary work and discussion – as well as care for those he called the “least and the rejected”.
He explained he had chosen the name Leo after a 19th-century Pope known for his teaching on social justice.
The new Pope also suggested the development of artificial intelligence and other advances meant the church was necessary today for the defence of human dignity and justice.
He is due to hold an audience with the media on Monday ahead of his inauguration next Sunday.
As part of that mass he will deliver a homily in the presence of numerous heads of state and dignitaries.
The 69-year-old is the 267th occupant of the throne of St Peter, and the first American to become a pontiff. He will lead members of the Catholic Church’s global community of 1.4bn people.
Born Robert Francis Prevost in Chicago, he worked for many years as a missionary in Peru before being made an archbishop there. He also has Peruvian nationality.
Although Leo was born in the US, the Vatican described him as the second pope from the Americas. Pope Francis, from Argentina, was the first.
Pope Leo is widely seen as a moderate who can offer “continuity” and “unity” following the death of his predecessor last month.
The new pontiff is believed to have shared Francis’ views on migrants, the poor and the environment.
In his first speech he told the crowds he wanted “to walk together with you as a united Church searching all together for peace and justice”.
