ROME — Pope Francis joined other religious leaders Tuesday at Rome’s Colosseum to make a plea for peace and ending what they called the “nuclear nightmare” as fears intensify that Russia’s war against Ukraine may see atomic weapons used.

A ceremony at the ancient Roman arena capped a conference on promoting world peace. The Sant’Egidio Community, a Catholic charity with close ties to the Vatican, organized the three-day event in Italy’s capital.

In remarks to participants, Francis noted that Pope John XXIII urged government leaders exactly 60 years ago, during the U.S.-Russian Cuban missile crisis, to spare the world from a nuclear holocaust.

“Sixty years later, these words still impress us with their timeliness,″ Francis said. ”I make them my own.”

“Today peace has been gravely violated, assaulted and trampled upon, and this in Europe, on the very continent that in the last century endured the horrors of two world wars.” the pope lamented.

“Today, in fact, something we dreaded and hoped never to hear of again is threatened outright: the use of atomic weapons, which even after Hiroshima and Nagasaki continued wrongly to be produced and tested,″ Francis said, referring to the U.S. atomic bombing of the two Japanese cities in the waning days of World War II in the Pacific.

At various points in the war in Ukraine, President Vladimir Putin and other Russian officials have alluded to the possibility of using nuclear weapons to protect Russian territory, included annexed areas.

Despite wars in too many parts of the world these days, he said, “the plea for peace cannot be suppressed. It rises from the hearts of mothers; it is deeply etched on the faces of refugees, displaced families, the wounded and the dying,″ Francis said.

The ceremony ended with the reading aloud of a joint appeal for peace from the pope and other religious leaders. Francis and the religious representatives in attendance then signed the document.

“Before it’s too late, may negotiations capable of leading to just solutions for a stable and lasting peace, be activated,” the appeal states.

“Peace is holy. War can never be so. Humanity must put an end to wars, or it will be war to put an end to humanity,″ it reads.

The appeal included a call for the world to be “rid of the nuclear nightmare,” for the sake of future generations.

Edith Bruck, a Hungarian-born writer who lives in Rome and is a friend of Pope Francis, handed out copies of the document to young people at the ceremony.

At the start of Tuesday’s event, the pontiff circled the inside of the Colosseum in a wheelchair guided by aides for a prayer with fellow Christian leaders. Francis, 85, often uses a wheelchair because of a knee problem.

Outside the arena, other religious figures, including Rome’s chief rabbi, greeted him for the final speeches and appeal.