Around 350,000 pilgrims along with representatives from nations and religious bodies around the world attended as the ceremony steeped in tradition was held to inaugurate Benedict as leader of the world’s 1.1 billion Catholics.
The ceremony comes three weeks after the death of his predecessor John Paul II and two weeks after nearly two million people packed the streets around the Vatican for his funeral.
Dressed in shimmering golden vestments, Benedict told the crowd he was “a weak servant of God” and appealed for prayers to help him in the “enormous task that truly exceeds human capacity”.
He repeated his promise to continue John Paul’s policy of reaching out to Jews and “believers and non-believers alike”.
However his main message was a call to the faithful to “do all we can to pursue the path toward the unity you have promised”.
“My real program of governance is not to do my own will, not to pursue my own ideas, but to listen, together with the whole church, to the word and the will of the Lord,” said the 78-year-old German, speaking in Italian.
Applause echoed around the colonnaded square as flag-waving pilgrims interrupted his powerful sermon more than 40 times, chanting “Benedict, Benedict,” at the end of the speech.
He was draped in the pallium, a narrow stole of white wool, and the Fisherman’s Ring bearing the image of first Pope Saint Peter was placed on his right ring finger.
The pontiff, who made numerous references to John Paul, ended his speech by quoting the late pope’s words at his 1978 inauguration: “do not be afraid,” assuring that by trusting in God “we lose nothing, absolutely nothing, of what makes life free, beautiful and great.”
After the almost three-hour long mass, the beaming Pope Benedict was driven around the square in an open-top car, with pilgrims clamouring to congratulate him.
Formerly Cardinal Joseph Ratzinger, the Vatican’s austere and stern guardian of Catholic orthodoxy, the new pope looked to soften his image and smiled often, in a bid to come across as more relaxed than his reputation.
Security was as tight as it was for John Paul’s funeral, with Rome shutting its airspace, closing roads and preparing anti-aircraft missiles and a NATO plane guarding against attack.
The Pope was elected in a secret conclave of the church’s 115 voting cardinals after just three rounds of voting on Tuesday.
He is the 16th pontiff to take the name Benedict.
Before the mass, he visited the tomb of the St Peter, who is buried in the crypt beneath the main altar of the vast basilica.
Benedict’s brother Georg, 81, who is a priest, was also present.