Pope Francis used his inauguration today to plead on behalf of the poor, the elderly and "the last we think about".
Around 200,000 people attended the Mass in St Peter's Square, including global leaders and dignitaries.
The Duke and Duchess of Gloucester were there to represent the Queen, while Ken Clarke and Baroness Warsi represented the British Government.
Pope Francis spoke of the need to "be protectors of God's gift" and not cause the "destruction" of nature.
"It means protecting all creation, the beauty of the created world," he said in his homily.
"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 faithful were called to forsake the hatred, envy and pride that "defile our lives" and instead be people of "goodness" who treat others with "tenderness".
"Let us not allow omens of destruction and death to accompany the advance of this world," he said.
The new Pope was the first to choose the name Francis, after Francis of Assisi who shunned a life of privilege to live among the poor.
During the Mass, Francis spoke of the calling of a pope to be close to the "poorest, the weakest, the least important, those who Matthew lists in the final judgement on love: the hungry, the thirsty, the stranger, the naked, the sick, those in prison".
At the start of the Mass, Francis received the fisherman's ring, engraved with the image of St Peter holding two keys, and a lambs' wool pallium symbolising his role as shepherd of the flock.
Earlier, the Pope toured St Peter's Square in an open-topped Popemobile, stopping to bless a baby and disabled man.