Happiness cannot be downloaded like a smartphone app, Pope Francis told teenagers on Sunday as part of a weekend-long celebration of young people in the Vatican.

Around 100,000 people aged 13-16 gathered in St Peter's Square for the three-day event that is tied to the Vatican's year of mercy festivities. As part of the weekend youngsters had an opportunity to admit their sins to the 79-year-old pontiff after he spontaneously joined 150 priests hearing confessions on Saturday.

"Your happiness has no price. It cannot be bought. It is not an app that you can download on your phones, nor will the latest update bring you freedom and grandeur in love," he said at a special mass on Sunday.

"Don't believe those who would distract you from the real treasure, which you are, by telling you that life is beautiful only if you have many possessions," said Francis.

"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 when you wear the latest fashions."

Sunday's mass followed a Saturday evening concert in Rome's Olympic stadium. Francis sent a video message to the thousands gathered to hear rock and rap musicians: "Just remember that if Jesus is not in your life, it is as though there is no signal."

One teenager who had had confession with the Argentine Pope told La Republica newspaper afterwards: "It was the most emotional day of my life."

The pontiff's informal tone is cited as one reason he is so popular with Catholic young people. This is combined with the Vatican's active presence on social media. Last month Pope Francis joined Instagram and he is judged to be the most influential tweeter in the world.