Pope Benedict XVI bestowed his final Sunday blessing of his pontificate on a cheering crowd in St. Peter's Square, explaining that his waning years and energy made him better suited to the life of private prayer he soon will spend in a secluded monastery than as leader of the Roman Catholic Church.

On Thursday evening, the 85-year-old German-born theologian will become the first pope to have resigned from the papacy in 600 years.

Sunday's noon appearance from his studio window overlooking the vast square was his next-to-last appointment with the public of his nearly eight-year papacy. Tens of thousands of faithful and other admirers have already asked the Vatican for a seat in the square for his last general audience Wednesday.

Perhaps emotionally buoyed by the warm welcome, thunderous applause and the many banners reading "Grazie" (Thanks) held up in the crowd estimated by police to number 100,000, Benedict looked relaxed and sounded energized, in sharp contrast to his apparent frailty and weariness of recent months.

Nuns wave at Pope Benedict XVI from Saint Peter's Square on Sunday. (Eric Gaillard/Reuters)

In a strong and clear voice, Benedict told the pilgrims, tourists and Romans in the square that God had called him to dedicate himself "even more to prayer and meditation," which he will do in a monastery being renovated for him on the grounds behind Vatican City's ancient walls.

"But this doesn't mean abandoning the church," he said, as many in the crowd looked sad at his approaching departure. "On the contrary, if God asks me, this is because I can continue to serve [the church] with the same dedication and the same love which I have tried to do so until now, but in a way more suitable to my age and to my strength."

The phrase "tried to" was the pope's adlibbed addition to his prepared text.

Pope acknowledges followers 'gratitude, affection'

Benedict smiled in pleasure at the crowd after an aide parted the white curtain at his window and he gazed at the people packing the square, craning their head for a look at him. Giving greetings in several languages, he gratefully acknowledged what he said was an outpouring of "gratitude, affection and closeness in prayer" since he stunned the church and its 1.2 billion members on Feb. 11 with his decision to renounce his papacy and retreat into a world of contemplation.

"Prayer is not isolating oneself from the world and its contradictions," Benedict told the crowd. He said he had heard God's call to prayer, "which gives breath to our spiritual life" in a special way "at this moment of my life."

Heavy rain had been forecast for Rome, and some drizzle dampened the square earlier in the morning. But when Benedict appeared, to the peal of church bells as the clock struck noon, blue sky crept through the clouds.

"We thank God for the sun he has given us," the Pope said.

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 yet for conclave

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."