Pope Leo XIV, the first American pope, presided over his inaugural Mass in St. Peter’s Square on Sunday in front of a crowd of tens of thousands. World leaders including Vice President JD Vance and top church officials sat in the Roman sun for the outdoor service that officially installed Leo as leader of 1.4 billion Roman Catholics.

Evoking some of the messages of his predecessor, Pope Francis, who died in April, Leo, used the occasion to plead for peace in Ukraine, Gaza and Myanmar. As he became the 267th head of the Roman Catholic Church, Leo said that his priority was for the church to hold to its mission of evangelization, another echo of Francis, and chart a path that included as many people in the church as possible.

It “is never a question of capturing others by force, by religious propaganda or by means of power,” but through love, he said.

Vice President JD Vance, Secretary of State Marco Rubio, President Volodymyr Zelensky of Ukraine, Prime Minister Mark Carney of Canada and Prime Minister Anthony Albanese of Australia were among the world leaders who had flown to Rome to attend the Mass. The pope shook hands with dignitaries, including Mr. Vance and Mr. Zelensky, whom he was expected to meet with later on Sunday.

Mr. Vance’s visit to the Vatican has been seen as a potential reset for the strained relationship between the papacy and conservative Catholics in the United States. Four weeks ago, Mr. Vance met with Francis, who had publicly chastised his theology and position on immigration. On Sunday, he greeted a leader who appears to share many of his predecessor’s views, including direct criticism of Mr. Vance in a now-deleted social media account associated with his name.