Benedict will become the first pointiff to step down in 600 years Thursday. The pope cited his age and diminishing strength as the main reason for his resignation. He said he felt incapable of adequately fulfilling the ministry entrusted to him.
His appearance Sunday from his studio window was his next-to-last public liturgy of his nearly eight-year papacy.
The 85-year-old said he plans to spend his last years in prayer, meditation, and seclusion on Vatican City grounds. Benedict told the pilgrims, tourists and Romans in the square that God had called him to dedicate himself "even more to prayer and meditation".
"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 it (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."
Benedict went on to thank his tens of thousands of faithful admirers in the square and the church's 1.2 billion members in several languages.
He acknowledged the world's outpouring of "gratitude, affection and closeness in prayer."
No date has yet been set for the start of the conclave of cardinals, who will vote in secret to elect Benedict's successor.
The Associated Press contributed to this report.