He was seated on Sunday in a great white and gold chair under a colourful tapestry depicting the Biblical miracle of the fish - hung from the balcony where his election as the head of the world's 1.1 billion Catholics was proclaimed last Tuesday.
The inaugural mass was concelebrated by 150 cardinals with squat white mitres and off-white cloaks with sparkling gold markings over white vestments worn to mark the sacred nature of the occasion.
The 78-year-old Benedict, wearing a gold mitre and cloak over his white vestments and carrying the crucifix so long associated with his predecessor John Paul II's public outings, waved to the roaring crowd as he emerged into the sunlight gracing St Peter's Square.
As with John Paul II's funeral on 8 April, the square was a patchwork of colours, with a purple swath of bishops, a white section of nuns and priests and the black of dignitaries from dozens of countries.
Swiss Guards provided the colourful face to a security operation involving some 10,000 mostly plain clothed officers dotted around the crowd approaching some 500,000.
In a break with past papal inaugurations - when all attending cardinals took part in a rite of allegiance, kneeling before the pope and kissing his hand - on Sunday the ceremony was performed by a selection of 12 people, equal to the number of apostles.
Selected to reflect the diversity of the Church, they included Cardinal Angelo Sodano, the Vatican secretary of state, and the cardinal who proclaimed the pope's election, Jorge Arturo Medina Estevez of Chile, but also a nun, a monk, a young Korean couple and their child, and two young people, one from Sri Lanka and another from the Democratic Republic of Congo.
The late Pope John Paul II was
laid to rest on 8 April
It was a relaxed, intimate moment during which the smiling pope had brief chats with each, although the Congolese, kneeling before the pope and kissing his hand, seemed to be speechless with awe.
The pallium, a white tie-like garment with five red crosses on
it, symbolises the pope's role as shepherd of God's flock, and the Fisherman's Ring, bearing an image of St Peter and the fishermen's net, recalls when Jesus revealed his divinity to Peter in the Biblical miracle of the fish.
The two objects - the ring set in an open, gold-encrusted case, and the pallium lying on a silver tray - had been borne in a solemn procession from St Peter's tomb in the heart of the basilica where Benedict XVI paused in prayer before the start of the mass.
Calm and nervous
The pope, who had appeared alternately calm and nervous, broke into his first full smile as Sodano bestowed the ring to a riot of applause from the throngs, thousands of whom were following the ceremony on giant screens set up in the square and the main artery leading up to it.
He seemed to grow in confidence as he delivered his homily, a message frequently interrupted by applause, in which he asked for prayers to help him, a "weak servant of God," in the "enormous task which truly exceeds all human capacity."
"I am not alone," he said. "I do not have to carry alone what,
in truth, I could never carry alone.