Hundreds of thousands of young people filled a vast field on the outskirts of Rome on Saturday to see Pope Leo, in the largest event yet of the new Catholic pontiff's tenure, as part of a special weekend aimed at energising the Catholic youth.

Young people from more than 146 countries, some wearing colourful bandanas to ward off the hot summer sun, were pressed against fences in the Tor Vergata field as Leo toured the crowd in his white popemobile in the late afternoon.

The pope, smiling broadly, waved, offered blessings and occasionally caught small stuffed animals and national flags thrown by the youth as he passed by.

"Dear young people ... my prayer for you is that you may persevere in faith, with joy and courage," Leo said in remarks later to the crowd.

"Seek justice in order to build a more humane world," he said. "Serve the poor, and so bear witness to the good that we would always like to receive from our neighbours."

Many of the youth attending the event with Leo spent all day waiting in the field in heat approaching 30 degrees Celsius to see the pope.

Many pilgrims are expected to sleep in the field overnight, waiting for a second chance to see Leo on Sunday morning, when the pope is due to celebrate a Catholic mass.

The weekend events are tied to the ongoing Catholic Holy Year, which the Vatican says has attracted some 17 million pilgrims to Rome since it started at the end of 2024.

Leo, the first US-born pope, was elected on May 8 by the world's cardinals to replace the late Pope Francis. (Reuters)