The USA's Vice President JD Vance and Foreign Minister Marco Rubio, as well as Ukraine's President Volodymyr Zelenskyj and Germany's Federal Chancellor Friedrich Merz, are some of the hundreds of dignitaries present.
From Sweden, Crown Princess Victoria is participating, among others.
The ceremony is led by the new Pope, and in his sermon, he urges the Church to be a transformative force in a world marked by division and hatred.
In our time, we still see far too much discord, far 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 says.
Ride in the Pope Mobile
At the same time, Pope Leo warns against "locking ourselves into our small groups".
We are called to offer God's love to all, in order to achieve the unity that does not cancel out differences but values every human being's personal story – and every people's social and religious culture.
Before the Mass, Leo took his first ride in the Pope Mobile. On St. Peter's Square, the gathered crowds cheered and waved, among other things, Peruvian and American flags.
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".
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.
