The Pope has delivered an emotional last Sunday prayer in St Peter's Square, saying God had told him to devote himself to prayer but assuring supporters he would not "abandon" the Church.
Tens of thousands of supporters turned out for Benedict XVI's historic prayers ahead of his formal resignation on Thursday, often interrupting the Pope with their clapping, cheering and chanting.
"The Lord is calling me to climb the mountain, to dedicate myself even more to prayer and meditation. But this does not mean abandoning the Church," the Pope told the crowd from the window of his residence in the Apostolic Palace in the Vatican.
"If God is asking me to do this it is precisely so I can continue to serve with the same dedication and love as before but in a way that is more appropriate for my age and for my strength."
The 85-year-old leader of the world's 1.2 billion Roman Catholics has said he is stepping down because he no longer has the strength of mind and body to carry on.
His shock resignation ended an eight-year pontificate dominated by the priest child sex abuse scandal and efforts to counter rising secularism in the West.
He thanked the crowd with a final unscripted call, telling them: "We will always be close."
The Vatican and Rome police estimated the numbers at more than 100,000 people - many times more than usually attend the traditional Sunday prayer.
There was tight security in and around the Vatican, with more than 100 police officers and snipers on surrounding buildings with the security being seen seen as preparation for the Pope's final general audience in St Peter's on Wednesday.
The Vatican has said Benedict will retire to the papal summer residence of Castel Gandolfo near Rome for the next two or three months while a former monastery inside the Vatican is renovated.
Benedict has said he will live "hidden from the world" but Vatican spokesman Federico Lombardi has said he could provide "spiritual guidance" to his successor and would probably continue to publish his theological research.