The US Vice President JD Vance and Foreign Minister Marco Rubio, as well as Germany's Federal Chancellor Friedrich Merz and Sweden's Crown Princess Victoria, were among the hundreds of dignitaries present.
Ukraine's President Volodymyr Zelenskyj was also in attendance, and later on Sunday, he and Pope Leo in an audience.
In his sermon, Pope Leo referred to the Ukraine war.
The suffering Ukraine is waiting for negotiations to finally reach a just and sustainable peace, said the American.
Ride in the Popemobile
He also urged the Church to be a transformative force in a world marked by division and hatred.
In our time, we still see too much discord, too many wounds caused by hatred, violence, prejudices, fear of differences, and an economic paradigm that exploits the earth's resources and marginalizes the poorest, he said.
Pope Leo also warned against "locking ourselves into our small groups".
We are called to offer God's love to all, to achieve the unity that does not erase differences but values every person's personal story – and every people's social and religious culture.
Before the Mass, Leo took his first ride in the Popemobile. On St. Peter's Square, the gathered crowds waved, among other things, Peruvian and American flags.
The First US Pope
Inacia Lisboa, originally from Cape Verde but living in Rome, said she got up early to get a good spot to meet a man who had already "won a place in her heart".
When asked what she wanted to hear from the Pope, she said:
The first thing is that he prays for us all, for peace in the world. We really need that.
Leo XIV was elected Pope on May 8, after the previous Pope Francis had passed away at the end of April. Leo is the first American Pope in history, but he has spent large parts of his life in Peru.
