"I renounce the ministry of Bishop of Rome, Successor of Saint Peter, entrusted to me by the cardinals," the frail pontiff said in a whisper.
The leader of 1.2 billion Catholics admitted he no longer had the "strength of mind and body" to keep up with a modern world "subject to so many rapid changes and shaken by questions of deep relevance for the life of faith."
The simplicity of the words contrasted with Cardinal Angelo Sodano's reply to the pope that his action was "like a lightning bolt in a clear blue sky".
The scene was modest and startlingly sincere -- in keeping with the sober style of a German theologian who has struggled to get a grip on the burning issues of the day for the Catholic Church in his eight-year pontificate.
The momentous news, that will make the pope only the second pontiff to resign of his own free will in 2,000 years of history, reverberated around the world.
But the pope has continued unruffled with his usual schedule, urging an end to "rivalry" within the Church at his last public mass on Wednesday and calling for "spiritual renewal" at an audience with Roman priests on Thursday.
"I will always be close to all of you and I am sure you will remain close to me even though I will be hidden from the world," he said.
"The pope is living through these days calmly even though they're very emotional and packed," said Vatican spokesman Federico Lombardi.
At the Ash Wednesday mass, which marks the start of a period of penitence for Christians before Easter, the pope appeared relieved and smiling.
When waves of applause broke out in St Peter's Basilica, the pope cut short the outpouring of emotion with a curt: "Let us return to prayer".
After a week-long spiritual retreat starting on Sunday, the pope will have only very few public engagements before he formally steps down on February 28.
Benedict will receive Italian President Giorgio Napolitano on February 23, celebrate his weekly Angelus prayer on February 24 and hold a final audience in St Peter's Square with tens of thousands of followers on February 27.
The Vatican has received 35,000 bookings so far for the general audience but is expecting many more to come from around the world and Rome city authorities have set up a special task force to make security arrangements.
The event is specifically not intended as any grand institutional ceremony with world leaders or celebrities according to the pope's wishes.
On the 28th at 1600 GMT, a helicopter will whisk the pontiff away to his temporary abode, the papal summer residence of Castel Gandolfo near Rome.
The pope will dine, greet pilgrims from his window and pray in a private chapel for the poignant final moments of his eight-year pontificate. At 1900 GMT, he will no longer be pope and will begin a life of quiet contemplation.
Asked why the pope had chosen 8:00 pm instead of midnight for the resignation, Lombardi said simply it was "the end of a normal working day".
"We should not expect any very formal or very solemn act. There will be no great ceremony with a great speech on the 28th," Lombardi said.
"He has already said he is renouncing his post and that is enough," he said.
The pope will eventually reside in a former monastery within the Vatican grounds, setting up an unprecedented situation for the Church in which a pope and his predecessor will live within a stone's throw of each other.
The exact role and title that the pope will have after he steps down is still undetermined although Lombardi has indicated Benedict who will revert to his name of Joseph Ratzinger could provide spiritual guidance to his successor.
"He will remain Benedict for us, that cannot change, but if we bump into him in the street I'm not sure what we'll call him," Lombardi said this week.
There have even been questions about whether the former pope will be allowed to wear white like a pontiff or will have to revert to the black cassock.
Mindful of the precedent of two or even three popes reigning at the same time, senior prelates say the prospect of a pope and his predecessor living in the Vatican will be untenable and Ratzinger will eventually have to move.
A return to Germany or a move to another monastery have been mooted.
But Lombardi has insisted the pope "never even considered any alternative" arrangement and will live out his old age in the Mater Ecclesiae monastery together with his housekeepers and his private secretary Georg Gaenswein.
"The pope has very modest needs. People who know him know that well."