Pope Francis has promised to bring his followers hope by service at his inauguration in front of a crowd of 150,000 in Rome's St Peter's Square.
Gathered under bright sunshine, the 266th pontiff called for Christians to defend the weakest in society and protect their faith in his keynote address at the centre of the ceremony, which will have been watched around the globe.
He had earlier been greeted by passionate scenes in front of the Basilica as he arrived, beaming, in the Popemobile.
Pope Francis left the vehicle to greet some of followers at the front of the huge crowds, which sported a sea of banners declaring support for the new incumbent from a myriad of nationalities who had descended on the Italian capital for the historic occasion.
The Argentinean adopted a more humble approach than the inauguration eight years ago of his predecessor Benedict XVI, whose installation as the head of the Catholic church was marked in baroque splendour.
In the traditional homily to the crowd, Pope Francis paid tribute to Benedict, sending him "prayers, affection and thanks".
Regarding his own role as the Bishop of Rome, Pope Francis said he would bring hope by using the "authentic power of service".
(The Church's mission) 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.
He added that demonstrating kindness and compassion is "not a sign of weakness".
In keeping with the Church's ambition to broadcast his message to the widest possible audience, the key messages from his homily were tweeted soon afterwards from the Pope's official account.
Let us keep a place for Christ in our lives, let us care for one another and let us be loving custodians of creation.
True power is service. The Pope must serve all people, especially the poor, the weak, the vulnerable.
Baroness Warsi joined Cabinet colleague Kenneth Clarke in representing the British government at the inauguration and paid tribute to the new pope.
Pope Francis inherits the papacy at a difficult time in its history.
The Church has been mired in scandals over priests' sexual abuse of children and the leak of confidential documents alleging corruption and rivalry between cardinals inside the Church government or Curia.
Many of the cardinals he stood to greet during today's ceremony - again demonstrating a humbleness towards his position - reportedly may not survive a shake-up of the frontline of the Church.
The closest the inauguration came to controversy was the attendance of Zimbabwe President Robert Mugabe, who is banned from the European Union, but free to travel to the Vatican.
The international pariah was warmly greeted among a succession of world leaders and dignitaries meeting the newly inaugurated pontiff inside St Peter's Basilica.