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".