Pope Francis celebrated Sunday Mass for tens of thousands of teenagers and offered them this piece of advice: acting tough like "heroes in films" or following the latest fashions will not make you happy.

Boys and girls between the ages of 13 and 16 gathered at the Vatican, including some 70,000 from Italy, as well as about 600 from Germany, the United States, the pope's home nation of Argentina and elsewhere, for the Young People's Jubilee, running from Saturday to Monday.

"Be sceptical about people who want to make you believe that you are only important if you act tough like the heroes in films or if you wear the latest fashions. Your happiness has no price," Francis said during an open-air Mass in St Peter's Square.

Francis, who said in the past he does not know how to use a computer, tried to adapt his language to his audience: he told them that happiness "is not an app that you can download on your phones, nor will the latest update bring you freedom and grandeur in love."

On Saturday, the 79-year-old Francis used a similarly unorthodox metaphor in a video message, broadcast ahead of a rock concert organized for the youth jubilee at the Stadio Olimpico, Rome's main football venue.

Holding a smartphone in one hand, the pope said living disconnected from Jesus was as frustrating as not being able to call up friends when there is no network coverage. "Remember that if Jesus is not in your life, it is as if there is no service," he said.

During Sunday Mass, Francis advised young people to beware of possessive love, excessive freedom and materialism.

Defining freedom as "the gift of being able to choose the good," Francis told his young audience that they "have to be able to say 'no'" to anyone suggesting that "freedom means doing whatever you want."

Sunday was also the day set by the pope for a fundraising campaign for Ukraine civil war victims. He has urged Catholic parishes all over Europe to give "generous" donations to help refugees and internally displaced civilians.

This weekend's youth jamboree at the Vatican was part of the Jubilee of Mercy, an 11-month festival ending in November. Francis has called it to offer Catholics a special chance to make pilgrimages and seek a general pardon for their sins.

In a spontaneous gesture on Saturday, the pontiff turned up in St Peter's Square and helped 150 priests who were giving confession to young people, sitting down on a chair and administering the sacrament to 16 teenagers.

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