World leaders and hundreds of thousands of people have celebrated Pope Francis' inaugural mass at St Peter's Square in the Vatican City.
Nearly 200,000 people attended the mass on Tuesday, which began with a tour of St Peter's Square by the former Cardinal Jorge Mario Bergoglio, 76, who was elected last week.
"I would like to ask all those who have positions of responsibility, let us be protectors of creation, protectors of God's plan inscribed in nature, protectors of one another and of the environment," the pope said in his homily.
At the ceremony, the 265th successor to St Peter received from his cardinals the papal pallium, a lambswool strip of cloth that symbolises the pope's role as a shepherd to the Catholic flock.
The "Fisherman's Ring" bestowed on him by Angelo Sodano, dean of the college of cardinals, is a personalised signet ring traditionally worn by popes in honour of St Peter - a fisherman.
Al Jazeera's Paul Brennan, reporting from the Vatican City, said the ceremony "symbolised a wedding" between the pontiff and the Catholic church.
"The ring has this spousal imagery to it, so that it is forever his bound to the church of Rome," our correspondent said.
The son of an Italian emigrant railway worker from a working-class quarter of Buenos Aires, the new pope has been effusive in a way that is unusual in the Vatican, kissing pilgrims and doing impromptu walkabouts.
Al Jazeera's Hoda Abdel-Hamid, also reporting from the Vatican City, said the biggest security challenge came about when the pope toured the square.
"He refused to have the bullet-proof [popemobile], he actually toured the square in an open roof," Abdel-Hamid said.
An estimated 3,000 officers including snipers on the rooftops were deployed during the event.
Despite criticism at home for allegedly failing to speak out against the excesses of Argentina's military rule, the former archbishop of Buenos Aires has already won hearts in Rome with a disarmingly informal style.
The former Cardinal Bergoglio was a surprise choice at a conclave of cardinals to replace 85-year-old Benedict XVI, who last month brought a sudden end to a papacy that had often been overshadowed by scandal.
The pope said he chose his papal name in honour of the medieval Italian priest, St Francis of Assisi, and has called for a "poor Church for the poor," warning the world's cardinals against pursuing worldly glories.
The arrival of the world leaders presented Francis with one of his first diplomatic challenges after his compatriot, President Cristina Kirchner, asked him to mediate in a dispute with Britain over the Falkland Islands.
The Chinese government had also said it would not be sending any representatives after Taiwanese President Ma Ying-jeou said he was attending.
Zimbabwean President Robert Mugabe also flew in, sidestepping an EU travel ban over human rights abuses in his country that does not apply to the Vatican.
Latin America was heavily represented at the mass by the first non-European pope in nearly 1,300 years, with the presidents of Argentina, Brazil, Chile, Costa Rica, Mexico and Paraguay in attendance.
Church leaders have urged Francis to move quickly to reform the intrigue-filled Roman Curia, the central administration of the Roman Catholic Church, and his appointments in the coming weeks will be closely watched.
Francis has indicated that he will press for a friendlier faith that is closer to ordinary people and for social justice, although the moderate conservative is unlikely to change major tenets of Catholic doctrine.