He opened his inaugural Mass on March 19 in Vatican's St. Peter's Square before hundreds of thousands of people, including heads of state and leaders of other faiths.
"I confess to Almighty God and to you, my brothers and sisters, that I have greatly sinned, in my thoughts and in my words, in what I have done and in what I have failed to do, through my fault, through my fault, through my most grievous fault," Francis said. "Therefore I ask blessed Mary Ever-Virgin, all the angels and saints, and you, my brothers and sisters, to pray for me to the Lord our God."
Before the mass, Francis toured St. Peter's Square under bright sunshine in an open white jeep, abandoning the bullet-proof popemobile which was first built for and used by Pope John Paul II after an assassination attempt in 1981. Francis's predecessor, Benedict XVI, also traveled in the popemobile.
The new pontiff stopped frequently to greet some in the crowd gathered in the square, kissing babies and getting out at one point to bless a disabled person.
Francis received his newly minted gold ring symbolizing the papacy and a stole representing his role as shepherd of the world's 1.2 billion Catholics.
The stole, a liturgical woollen band worn around the neck, had been placed overnight on the tomb of St. Peter under the basilica's altar.
Among the world leaders attending the ceremony were Cristina Fernandez de Kirchner, the president of the pope's native Argentina, German Chancellor Angela Merkel, and U.S. Vice President Joe Biden, who along with Kirchner is a practicing Catholic.
Also present was Orthodox Church Ecumenical Patriarch Bartholomew from Istanbul. It's the first time the spiritual head of Orthodox Christians has attended a pope's inaugural Mass since the Great Schism between western and eastern Christianity in 1054.
Francis, the first South American and the first Jesuit pope, has already put his mark on the papacy, cutting down on the pomp of his predecessor Benedict and signaling that he wants the Catholic Church to focus on the poor and disadvantaged.
On March 19, the pope wore plain white vestments and black shoes, in contrast to the red loafers used by Benedict, and also shortened the Mass from three hours to two.
After the mass, Francis received the visiting political leaders in the basilica.