The process of choosing a successor to the 85-year-old pope is expected to begin on Friday with the Vatican sending out letters to Catholic cardinals from around the world to officially call on them to travel to Rome for the conclave.

The cardinals were also expected to begin informal consultations on the complex and largely secretive process of selecting a new pope, before the traditional rounds of talks open next Monday.

Not all of the details of how the conclave would be conducted this time around are clear. In part because it is the first time in several hundred years that a pope has stepped down, rather than remaining in office until his death.

A total of 115 cardinals - all under the age of 80 - are eligible to vote in the conclave to be held in the Sistine Chapel. No clear frontrunner for the job has emerged since Pope Benedict announced last month that he intended to step down. Cardinals who are older than 80 are not eligible for the conclave but may take part in general discussions leading up to the formal process. Those talks are meant to set out priorities for the future course of the church, following eight years under the German-born pope.

Pope Benedict spent much of his last day in the job on Thursday saying goodbyes to Vatican staff, cardinals, and members of the papal Swiss Guard.

Speaking to the cardinals in the morning, Benedict wished them well as they begin the process of selecting his successor.

"May the Lord show you what he wants. Among you there is the future pope, to whom I today declare my unconditional reverence and obedience," he said.

This was significant due to the fact that this will be the first time in history that a pope and a former pope will live side-by-side in the Vatican.

In the early evening, he left the Vatican for the last time as pope, flying to the 17th century papal retreat of Castel Gandolfo in the hills south of Rome.

After arriving at the retreat, the pope briefly appeared on the balcony of the palace and addressed the town's main square, which was packed with well-wishers.

"I am simply a pilgrim beginning the last leg of his pilgrimage on this Earth," the pope said before giving a final blessing.

"Grazie e buona notte," he said in Italian, which translates to: "thank you and good night."

Reaching out to the wider Catholic world, Benedict also posted a message on the micro-blogging website Twitter.

"Thank you for your love and support," Benedict wrote in his final tweet. "May you always experience the joy that comes from putting Christ at the center of your lives."

pfd/ccp (AFP, AP, Reuters)