Updated - Pope expresses gratitude, conclave after March 15

"Continue to pray for me, for the Church and for the future pope," he said in unscripted remarks at the start of his weekly general audience, one of his last public appearances before he resigns on Feb. 28.

The pope, who looked and sounded strong, was interrupted several times by thunderous applause from the some 8,000 faithful and tourists who packed the vast audience hall.

In brief, prepared remarks that mirrored those he read to stunned cardinals when he announced his decision on Monday, the pope said God would continue to guide the Church because it was much more than its earthly leader.

"I took this decision in full freedom for the good of the Church after praying for a long time and examining by conscience before God," he said.

He said he was, "well aware of the gravity of such an act but at the same time aware of not being able to carry out my (papal) ministry with the physical and spiritual force that it requires".

Benedict said he was sustained by the "certainty that the Church belongs to Christ, who will never stop guiding it and caring for it".

He said that "he felt almost physically" the affection and kindness he had received since he announced the decision.

Later on today, an Ash Wednesday Mass that was originally scheduled to have taken place in a small church in Rome, has been moved to St Peter's Basilica so more people can attend.

Unless the Vatican changes the pope's schedule, it will be his last public Mass. 

CONCLAVE AFTER MARCH 15

Meanwhile, the Vatican announced that the conclave to decide the successor to Pope Benedict will start as early as March 15.

The conclave, when cardinals gather to elect a new pope, will start between 15 and 20 days from when the papal seat is vacated on Feb. 28, Vatican Spokesman Father Federico Lombardi told a news conference.

Lombardi said Catholics should not be disoriented by Benedict's decision.