Pope Francis has vowed to embrace the world's most vulnerable and called on economic and political leaders to shun "destruction".
The first Jesuit Pope has been speaking at his inaugural mass in St Peter's Square. For the mass he wore plain white vestments, trimmed with gold and brown, and black shoes, in contrast to the luxurious red loafers that attracted attention under Pope Benedict. Pope Francis, formerly Argentine Cardinal Jorge Bergoglio, delivered his homily in Italian and began by focusing on Joseph and his role as protector - of Mary, Jesus and the Church. The pontiff, referring to the saint Francis of Assisi whose name he has taken, said the role of protector was not just a Christian one.
He said:
"It means protecting all creation, the beauty of the created world. It means respecting each of God's creatures and respecting the environment in which we live.
"It means protecting people, showing loving concern for each and every person, especially children, the elderly, those in need, who are often the last we think about."
The 76 year-old called on "all those who have positions of responsibility in economic, political and social life" to be protectors of creation.
He added:
"To protect creation, to protect every man and every woman, to look upon them with tenderness and love, is to open up a horizon of hope.
"Let us not allow omens of destruction and death to accompany the advance of this world."
Pope Francis also asked those listening not to be so quick to condemn the failures of others. Austin Ivereigh from Catholic Voices was in Vatican City for today's event and told Premier's Des Busteed during the News Hour that a mood of change is in the air:
More than 150-thousand people crammed into St Peter's Square to hear the Pope urge the world to be more forgiving and merciful.
Rt Revd Peter Brignall is the Catholic Bishop of Wrexham. He tells Premier why he thinks so many people turned out:
Pope Francis had begun his day by touring St Peter's Square in an open-topped Popemobile. At one point he stepped down from the vehicle and approached the barriers to bless a disabled man and spent time touring the square, waving to the pilgrims. One of the Pope's first duties was to go down to the tomb of St Peter to venerate it. He was then presented with his papal pallium made of lambs' wool - symbolising his role as shepherd of his flock - in the main square. He also received the "fisherman's ring" bearing the image of St Peter holding two keys. The ring is second hand and is made of silver-plated gold, not the solid gold worn by Benedict XVI. The Pope then received the obedience of the cardinals, and the Mass formally began. Representing the UK Government at the mass was minister for Faith and Communities, Baroness Warsi.
She said:
"For me what was quite beautiful was that I was surrounded, both where we were sitting but also amongst the crowds, there were people of all faiths there and I think his message was universal and was felt by people of all backgrounds."
The Duke and Duchess of Gloucester represented the Queen. Before the Mass, the Catholic leader of England and Wales spoke of his excitement over a fresh start for the Catholic Church.
The Archbishop of Westminster, Vincent Nichols, said:
"There are no tickets for this mass, anyone who wants to come may come; that's why the crowd will be so big and it is essentially the celebration of mass, and for mass the doors are always open."
Pope Francis was elected last week to take over from Benedict XVI, who became the first pontiff in 600 years to resign last month saying he felt he could no longer carry out his duties to the best of his ability.