Pope Benedict XVI held his last scheduled public Mass, for Ash Wednesday, in St Peter's Basilica.
He anointed the foreheads of the faithful with ashes, in a service attended by cardinals, bishops, monks, friars and pilgrims.
Earlier, he thanked the public for their "love and prayers" in his first public appearance since resigning.
Ash Wednesday begins Lenten season, a period of penitence before Easter.
Yesterday's afternoon Mass was relocated. The pontiff had been scheduled to celebrate Ash Wednesday at the small Sant' Anselmo church, then lead a procession to Santa Sabina Basilica on Rome's Aventine Hill.
The Vatican said the change to St Peter's was to accommodate the crowds, but it also saved the Pope the effort of the procession.
Earlier, the Pope was cheered by crowds as he entered and began speaking at his weekly general audience at its traditional venue, the audience hall in the Vatican.
Thousands of people gathered in the hall to greet him, giving him a standing ovation as he arrived, and cheering as he began and finished speaking.
Looking tired and drawn, Pope Benedict thanked the crowd for their warm greeting and their sympathy.
"Thank you for the love and prayer with which you have accompanied me... Keep praying for me, for the Church and for the future pope," he said.
He said he was aware of the gravity of his decision to resign but also of his declining strength, and added that he was certain the Church would sustain him with prayer.