Addressing crowds in St. Peter's Square on Sunday, Pope Leo XIV noted the 80th anniversary of the end of World War II and -- echoing his predecessor Pope Francis -- declared, "Never again war."

Leo urged an end to the ongoing wars in Ukraine and the Middle East. "I carry in my heart the suffering of the beloved Ukrainian people," he said. "Let us do everything possible to reach a genuine, just and lasting peace as soon as possible. May all prisoners be freed and children be able to return to their families."

"I am deeply saddened by what is happening in the Gaza Strip," he added. "Let the fire cease immediately. Let humanitarian aid be provided to the exhausted civilian population and let all hostages be freed."

"On the other hand, I welcomed with satisfaction the announcement of the ceasefire between India and Pakistan, and I hope that through the upcoming negotiations we can soon reach a lasting agreement," Leo continued.

-ABC News' Phoebe Natanson

Pope Leo XIV addressed crowds at the Vatican on Sunday, asking those gathered to pray for the priesthood.

The new pope spoke to the crowd to give the Regina Caeli prayer. Police estimated that there were around 100,000 people in St. Peter's Square to watch Leo's address.

-ABC News’ Phoebe Natanson

On his way back to the Vatican, Leo stopped by the Basilica of Saint Mary Major, and prayed before the tomb of Pope Francis, according to the Vatican press office.

He also prayed at the icon of the Virgin, Salus Populi Romani, that is the same icon Pope Francis used to visit before and after every foreign visit and in other occasions.

-ABC News' Phoebe Natanson

In an instant, Cardinal Robert Prevost became the most powerful and influential American Catholic when he was selected to be pope and renamed Pope Leo XIV on May 8.

But with the United States' standing in the world on edge with President Donald Trump in office, experts say many will be watching the first American to lead the Catholic Church to see how he walks the tightrope between Western politics and his papal duties.

Vatican experts told ABC News that Leo must thread that needle effortlessly, with a focus on leading from a global perspective.

"I suspect he will be scrupulous and careful to avoid the thought that he is an American and not a Christian view," the Rev. Stephen Spahn, an assistant teaching professor of theology at Loyola University Maryland, told ABC News. "He is going to be a bridge builder and build relationships with world leaders broadly."

At the same time, experts predicted Leo will not hesitate to speak out on U.S. matters from his seat when needed.

Spahn said cardinals have always come to their decisions knowing that the man who becomes pontiff will have to work within the geopolitical environment in pushing forward the Catholic Church's mission.

"It's definitely going to trail him. How could it not?" he said of the pope's Americanism. "But the cardinals have said they chose him without any regard to his nationality."

-ABC News' Ivan Pereira

Pope Leo XIV was met by hundreds gathered in the square in front of the Shrine of the Mother of Good Counsel in Genazzano, the Vatican said Saturday.

The pope entered the church and stopped to pray, first in front of the altar and then in front of the image of the Virgin Mary, where he recited Pope John Paul II’s prayer to the Mother of Good Counsel, according to the Vatican press room.

After reciting the Ave Maria and singing the Salve Regina, the pope turned to those in the church, greeting them and the people of Genazzano gathered outside: "I wanted so much to come here in these first days of the new Ministry that the Church has given me, to carry forward this mission as the Successor of Peter."

At the end of his visit, the Vatican said Pope Leo appeared at the door of the basilica and said a few words, expressing the joy of being able to come to pray to the Mother of Good Counsel, calling it "such a great gift" for the people of Genazzano, from which also comes a great responsibility: "Just as the Mother never abandons her children, you must also be faithful to the Mother."

The pope also greeted the young people and the young at heart: "We are all!"

Finally, before leaving the sanctuary, he blessed those present.

On his way back to the Vatican, the press office said Pope Leo went to the Basilica of Saint Mary Major, where he stopped to pray before the tomb of Pope Francis and the icon of the Virgin, Salus Populi Romani.

-ABC News’ Phoebe Natanson