Pope Francis officially began his papacy today with a pledge to serve the poorest and weakest in society in a multi-lingual Mass that celebrated the Eastern and Western traditions of the Church.

At the Inauguration Mass six days after his election as pope, Francis said that while the ministry of the bishop of Rome involved a certain power, "Let us never forget that authentic power is service, and that the Pope too, when exercising power, must enter ever more fully into that service which has its radiant culmination on the Cross."

Delivering his homily on the Feast of St Joseph, in a sunny St Peter's Square, which was filled with hundreds of thousands of people waving flags from all over the world, the Pope continued: "[The Pope] must be inspired by the lowly, concrete and faithful service which marked Saint Joseph and, like him, he must open his arms to protect all of God's people and embrace with tender affection the whole of humanity, especially the poorest, the weakest, the least important, those whom Matthew lists in the final judgment on love: the hungry, the thirsty, the stranger, the naked, the sick and those in prison [Mt 25: 31-46]. Only those who serve with love are able to protect!"

Pope Francis' voice rose passionately as he listed those in need. Otherwise, his message was striking for its simplicity, directness and gentle tone. He repeatedly called for the protection of all creation and cited the example of Saint Francis of Assisi, the inspiration behind his choice of papal name.

The Gospel reading was intoned in Greek, and in an historic first, the Ecumenical Patriarch of Constantinople not only attended the Mass but also prayed with the new pope in front of the tomb of St Peter before of the liturgy. The pair later exchanged the sign of the peace. This was the first time since the Great Schism of 1054 that the Patriarch of Constantinople had attended the inauguration of a Roman pope.

"They see in Pope Francis a person who has accepted a simple lifestyle, one that conforms to the Gospel," said Fr James Puglisi, who is the Minister-General of the Franciscan Friars of the Atonement and director of an ecumenical centre in Rome, told Vatican Radio. "And emphasising, first and foremost, that he is bishop of Rome, I think that was really important." The question of the primacy of the pope is a major sticking point in Catholic-Orthodox relations.

Archbishop John Sentamu of York represented the Anglican Communion because the Archbishop of Canterbury, Justin Welby is preparing for his enthronement on Thursday. The Queen was represented by her cousin, the Duke of Gloucester; Ken Clarke, minister without portfolio, and Lady Warsi, the minister for Faith and Communities, represented the Government.

Read Pope Francis' homily here.