Pope Francis urged Catholics to forgo worldly trappings and focus on essentials as he opened the season of Lent with a traditional Ash Wednesday Mass on one of Rome's historic seven hills.

He criticized people's tendency to lay bare their lives on social media, deploring "a world in which everything, including our emotions and deepest feelings, has to become 'social.'"

Instead, the faithful should enter their "inner chamber" to find time for quiet reflection and prayer, the 87-year-old pontiff said in a homily.

Lent is a 40-day period of penance that leads to Easter, the most important Christian festival, which celebrates the day on which Christians believe Jesus rose from the dead.

It represents the 40 days Jesus is said in the Bible to have spent fasting in the desert. During the season, Catholics are asked to fast, remember the needy and reflect on mortality.

"Life is not a play: Lent invites us to come down from the stage and return to the heart, to the reality of who we are," Francis said.

"Let us not be afraid to strip ourselves of worldly trappings and return to the heart, to what is essential."

He spoke at a service held in the Basilica of Santa Sabina on Rome's Aventine Hill, preceded by prayers in a nearby church and a procession of cardinals and bishops.

Mass goers, including the pope, had ashes sprinkled on their heads in the Ash Wednesday ritual that, for the world's more than 1.35 billion Catholics, serves as a reminder of mortality.