Pope Francis marked Ash Wednesday with a procession between two churches and urged Roman Catholics not to be obsessed with possessions.

"Often today, giving freely is not part of daily life, where everything can be bought and sold, where everything is calculated and measured. Giving without expecting anything in return helps us free ourselves from the obsession of owning things, from the fear of losing what we own, from the sadness of those who do not want to share their well-being with others."

Ash Wednesday is the day Christians are reminded of mortality and hear the phrase 'from dust you came and to dust you shall return' as ashes are smudged onto their foreheads.

Next Monday, the Pope will leave the Vatican with top aides for a week-long Lenten spiritual retreat at a religious institute south of Rome.

It is the first time in living memory that the retreat, a week of preaching and prayer, will be held outside the Vatican.