Pope Francis has told teenagers happiness can't be downloaded like a cellphone app.

Francis used the reference to connect to faithful aged 13-16 who are converging on Rome for a Holy Year weekend for teenagers.

During his homily Sunday in St. Peter's Square, he told a crowd of 100,000 that "happiness has no price" and is "not an app that you can download on your phones, nor will the latest update bring you freedom and grandeur in love."

A day earlier, in a video message to the teens' rally in a Rome stadium, Francis likened being out of contact with God through lack of love to being where there's no cellphone reception.

After Mass, dozens of white-robed priests surrounded Francis, many snapping selfies with him.