Pope Benedict XVI is due to hold what is expected to be his last public Mass later, for Ash Wednesday, in St Peter's Basilica, marking the beginning of Lent.
Earlier, the pontiff, who announced his shock resignation on Monday, was cheered by crowds as he entered and began speaking, at a weekly audience in a hall at the Vatican.
He said he was standing down for "the good of the Church", as he made his first public appearance since his unexpected announcement that he is to tender the first papal resignation since the middle Ages.
"Keep praying for me, for the Church and for the future Pope," he added, after a standing ovation from the thousands of people assembled in the audience hall.
He said he made the decision fully conscious of its gravity.
But he referred only briefly to his stepping down as Pope in 15 days' time, asking in particular for prayers for himself and his successor who will be chosen at a Vatican conclave next month.
Benedict XVI will later hold the Ash Wednesday Mass in St Peter's Basilica, marking the start of Lent, the most important festival in the Christian calendar.
It had been scheduled for a smaller venue on the Aventine hill, but the plans were changed in the aftermath of his announcement.
Meanwhile, Christian Aid's launched a brand new mobile app to help people get through Lent.
Using photos and thought provoking daily reflections the free 'Count Your Blessings' software is aiming to be a source of inspiration throughout the next 40 days.
The charity says users will be able to get daily notifications, hear uplifting stories from Christian Aid projects and take actions in solidarity with the world's poorest people.
Optional actions include giving 10p for every bedroom in your house as a reminder of how fortunate we are to have shelter and stopping to pray for people living in conflict.
Optimised for the retina display on iPhones, Ipads and HD Android devices, but available on non-HD devices as well, the app shows off some of the best images from around the world taken from the extensive Christian Aid photo archive.
Christian Aid's Church Resources Manager Claire Aston has told Premier's Marcus Jones on the News Hour why they've launched it:
The app is now available for download from the Apple Store and Google Play and also from www.christianaid.org.uk/lent.
Elsewhere, another Christian charity's hoping to raise millions for third world countries through Lent.
The Scottish Catholic International Aid Fund's urging people to put money saved through giving up towards helping the poor.
Paralympic gold medallist rower David Smith tells Premier why chips are off the menu for him for the next 40 days:
Meanwhile, three Welsh Bishops are taking up a tough Lent challenge which will see them give 40 talks over five weeks at eight different venues.
The Archbishop of Wales, the Assistant Bishop of Llandaff and the Bishop of Monmouth will be out and about in churches across South Wales almost every weekday night in the weeks leading up to Easter giving talks about the Bible. They're also inviting people to make their Lent resolution to join them for discussion.
Christians are also being encouraged to do no business with tax-dodging corporations such as Amazon and Starbucks for the duration of Lent, as a public witness against the sins of corporate tax avoidance.
Campaign group Christianity Uncut, a network of anti-capitalist Christians, suggest that a crackdown on tax avoidance is a better way of reducing the national deficit than cutting public services and the welfare state.