Tens of thousands of supporters turned out for the historic prayers ahead of the pope's 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 Catholics has said he will step 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.
"Holy Father, We Love You", read one banner seen in the crowd. One read: "Thank You, Your Holiness" and another said: "Dear Father, We'll Miss You".