The Pope has said he is resigning for the "good of the Church"
Pope Benedict XVI has celebrated his last Mass as Pope in St Peter's Basilica in Rome following the surprise announcement that he is to retire.
He presided over an Ash Wednesday ceremony marking the start of the Roman Catholic Church's season of Lent.
It came two days after the surprise announcement that the 85-year-old was to retire at the end of the month.
In his homily, he implicitly criticized recent infighting among clerics inside the Vatican.
He said the face of the Church had been marred by divisions and rivalry among the clergy, reports the BBC's David Willey, who was at the service.
Looking tired, the pontiff anointed the foreheads of the faithful with ashes in a service attended by cardinals, bishops, monks, friars and pilgrims.
The season of Lent is marked by fasting and acts of penitence for past sins, with worshippers marking a cross on their foreheads with wood ash to symbolise human mortality.
It is the most solemn season in the Church calendar that ends with Holy Week and Easter Sunday.
Earlier in the day, the Pope held his last but one general audience inside the Vatican, his first public appearance since he stunned the Church and wider world by saying he would retire at the end of the month.
He said he had made his decision only after long reflection.
"I am conscious of the gravity of my action," he said, receiving a standing ovation from 8,000 pilgrims from around the world gathered in the Vatican's main audience hall.
"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.
Next week the Pope will carry out no public engagements. He will enter a spiritual retreat and pray inside the Vatican.
The pontiff had originally 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.
But the Mass was relocated to St Peter's to accommodate the crowds, the Vatican said. The change also saved the Pope the effort of the procession.
The pontiff will continue with his usual agenda until the day he officially retires at the end of February, Vatican officials say.
By the end of Lent, in six weeks' time, there is expected to be a new pope.