Pope Francis has inaugurated his papacy at a Mass in Rome, calling on global leaders and all the people of the world to defend the poor and the weak.
Up to 200,000 people attended the Mass in St Peter's Square.
His homily focused on protection - of the environment, children, the elderly and those in need, who he said were "often the last we think about".
Francis was elected by a conclave of cardinals last week to take over from Benedict XVI.
Benedict became the first pontiff in 600 years to abdicate last month. Citing his age, 85, he said he could no longer continue in the post.
Pope Francis, formerly Argentine Cardinal Jorge Mario Bergoglio and now the first pontiff from the Americas, has since his election called for a "Church for the poor" and has struck an informal and spontaneous tone.
His chosen name, Francis, honours St Francis of Assisi, the 13th-Century son of an aristocrat who spurned a life of luxury to live with and for the poor.
Pope Francis' homily at the Mass began by focusing on Joseph and his role as protector - of Mary, Jesus and the Church.
Francis, 76, expanded the image, referring to Francis of Assisi and saying that 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."
Francis called on "all those who have positions of responsibility in economic, political and social life" to be protectors of creation.
"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," Francis said.
Without care for the environment and fellow humans, "the way is opened to destruction and hearts are hardened", he said.
"Let us not allow omens of destruction and death to accompany the advance of this world."
Francis said the pope himself must be inspired by the lowly - "the poorest, the weakest, the least important, those who Matthew lists in the final judgment on love: the hungry, the thirsty, the stranger, the naked, the sick, those in prison".
A new look Pope
Click a number to start
As Pope Francis celebrated his inaugural mass, his outward appearance signalled the style of his papacy which he has said aims to provide a "poor Church, for the poor".
Click on the numbers on the left to see how Pope Francis' choice of vestments and attire compare with those of his predecessor, Pope Benedict XVI.
Images: Reuters, AP
Papal stole 1 of 5
At his first appearance on the balcony of St Peter's, Pope Francis chose not to wear the red mozzetta (cape), a garment typically adopted by the newly-elected Pope.
He also chose not to wear the ornate Papal stole, except for the blessing.
Pope Benedict XVI in contrast wore both throughout his first blessing from the balcony.Return to introduction
Pectoral cross 2 of 5
The most recognised symbol of the Christian church around the world, the Pope wears a cross on the chest, usually suspended from the neck by a cord or chain.
Pope Francis opted for an iron cross rather than the ornate gold one preferred by his predecessor, in keeping with the more understated style of his pontificate.Return to introduction
Papal ring 3 of 5
A special ring, unique to each Pope, is presented as part of the inauguration mass.
Pope Francis selected a gold-plated, silver ring, which was initially designed for and held by the late Archbishop Pasquale Macchi, the longtime private secretary to Venerable Paul VI.
Benedict XVI wore a solid gold ring, crafted by Claudio Franchi and his family of goldsmiths.
Both rings show St. Peter casting out his net, symbolizing how popes are successors of the apostle Peter.Return to introduction
Vestments 4 of 5
Along with the ring a pallium is placed on the Pope during inauguration mass. This is a narrow scarf woven of white lamb's wool - symbolizing Jesus as the good shepherd.
The chasuble is the outermost vestment worn by clergy for the celebration of the Eucharist. Pope Francis wore mostly white, while Benedict XVI wore golden robes.
In contrast to Pope Francis' simple papal mitre, his predecessor's was ornate gold and with three levels of 'crowns' representing the powers of the Papacy in a simplified form.Return to introduction
Shoes 5 of 5
One of the most commented on aspects of Benedict XVI's inaugural mass was his red leather loafers, which were made by his personal cobbler, Adriano Stefanelli.
In contrast Pope Francis wore plain, dark leather shoes.
It is said that before he left Buenos Aires for Rome, as Cardinal Bergoglio, he was wearing a pair of shoes so shabby that friends insisted on buying him a new pair.Return to introduction
Francis had begun the 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.
Francis spent 20 minutes touring the square, waving to the pilgrims who flew flags and shouted: "Long live the Pope!"
Isaac Adroamabe from Uganda, said: "My first impression is that the Pope is very humble, and has taken the Church in his heart."
Seven-year-old Pietro Loretti, from Italy, said: "I like him because he loves the poor."
As the Mass began, Francis was presented with his papal pallium made of lambs' wool - symbolising his role as shepherd of his flock - and the fisherman's ring bearing the image of St Peter holding two keys.
With this, Francis officially began his office as the 266th pope.
Communion was distributed by some 500 priests throughout the crowd.
The Mass was co-celebrated by around 180 clergymen, including Adolfo Nicolas, the superior general of Pope Francis' Jesuit order.
The list of attendees also included Bartholomew, the Orthodox Patriarch of Constantinople.
He is the first Orthodox patriarch to attend a papal inauguration Mass since the two branches of Christianity split nearly 1,000 years ago.
The Pope later greeted the assembled dignitaries individually in St Peter's Basilica.
Hundreds of people also gathered in the early hours of Tuesday in Plaza de Mayo, the main square in Buenos Aires, to watch the Mass broadcast on giant screens set up outside the cathedral.
They erupted in joy as a call from the Pope, made an hour before his Mass, was played via loudspeakers.
He said: "My dear children, I know you're in the plaza. I know that you're making prayers. I need them very much."