Vatican City, Holy See (AFP) — Half a million young believers are expected in Rome next week for the Vatican’s “Jubilee of Youth” during a “Holy Year” announced by the Catholic church, organisers said Wednesday.

The event, for young adults aged 18 to 35, happens once every 25 years and will take place between July 28 and August 3.

It comes just over two months after Pope Leo XIV became the head of the Catholic church.

Students and young professionals will take part in a pilgrimage, culminating Sunday in a giant outdoor mass presided over by Leo, officials said at a Vatican news conference.

Archbishop Rino Fisichella said participants would come from 146 countries, of which 68 per cent will be from Europe.

“Many young people come from countries currently at war,” he said, such as pilgrims from Lebanon, Iraq, Burma, Ukraine, Israel, Syria and South Sudan.

The week will end next Saturday with a vigil with the pope in the eastern suburbs of Rome. Pilgrims will stay overnight until the final mass the next day.

The authorities in Rome said they were tightening security for the arrival of so many people.

“It is an event that, because of its importance, requires exceptional security measures,” Rome police chief Lamberto Giannini said.

“The international situation and various tensions require our utmost attention,” he added.

The event is taking place more than three years into Russia’s Ukraine invasion and as international condemnation of Israel’s actions in Gaza grows.

Pilgrims will also have to tackle the heat, as the Italian capital swelters in soaring temperatures.

Rome Mayor Roberto Gualtieri said authorities have prepared some 2,600 points of access to drinking water around the city, sprinklers, five million water bottles and medical assistance points.

The event was the “most important” in Italy “in terms of infrastructure technologies”, he added.

Some 13,000 people are involved in supporting the event including 3,000 civil protection volunteers, 1,000 police officers and more than 4,000 welcome staff.

During the last youth jubilee in 2000 more than two million people came to Rome at the invitation of Pope John Paul II.