Even as the cheering continued and shouts of "Long live the pope" went up in Italian and Spanish, the pontiff simply turned away from his window and stepped back down into the apartment, which he will leave Thursday, taking a helicopter to the Vatican summer residence in the hills outside Rome while he waits for the monastery to be ready.
A child in the crowd held up a sign on a yellow placard, written in Italian, "You are not alone, I'm with you."
No date has yet been set for the start of the conclave of cardinals, who will vote in secret to elect Benedict's successor.
"Now there will be two popes," said the Rev. Vilmar Pavesi, a Portuguese priest who was among the throngs in the square. "There will be the pope of Rome, the elected pope, and there will be the bishop emeritus of Rome, who will live the life of a monk inside the Vatican walls."
One Italian in the crowd seemed to be doing a little campaigning, hoisting a sign which mentioned the names of two Italian cardinals considered by observers to be potential contenders in the selection of the next pontiff.
Flags in the crowd represented many nations, with a large number from Brazil.
The cardinals in the conclave will have to decide whether it's time to look outside of Europe for a pope. The papacy was considered the realm of Italian prelates for centuries, until a Pole, John Paul II, was elected as pontiff in 1978, to be followed in 2005 by the German-born Benedict.
Crucially, Italian prelates have continued to run the behind-the-scenes machinery of the church's governance, and cardinals will likely be deciding what role the Italians might have played in a series of scandals clouding the central bureaucracy, including allegations of corruption and power-grabbing.
Benedict has not made any direct comment on details of the scandals.
In one of his last papal tweets, Benedict wrote Sunday in English: "In these momentous days, I ask you to pray for me and for the church, trusting as always in divine providence."
AP reporter Paolo Santalucia contributed to this report.