VATICAN CITY -- Pope Benedict XVI promised his "unconditional reverence and obedience" to his successor in his final words to cardinals Thursday, a poignant and powerful farewell delivered hours before he becomes the first pope in 600 years to resign.

In an unexpected speech inside the Vatican's frescoed Clementine Hall, the pontiff appeared to be trying to defuse concerns about possible conflicts arising from the peculiar situation of having both a reigning pope and a retired one.

He also gave a final set of instructions to the "princes" of the church who will elect his successor, urging them to be united as they huddle to choose the 266th leader of the world's 1.2 billion Catholics.

"May the College of Cardinals work like an orchestra, where diversity - an expression of the universal church - always works toward a higher and harmonious agreement," he said.

It was seen as a clear reference to the deep internal divisions that have come to the fore in recent months following the leaks of sensitive Vatican documents that exposed power struggles and allegations of corruption inside the Vatican.

The moment was as unique as Benedict's decision to quit, with the 85-year-old pope, wearing his crimson velvet cape and using a cane, bidding farewell to his closest advisers and the cardinals themselves bowing to kiss his fisherman's ring for the last time.

Some seemed to choke up at that moment, but the scene seemed otherwise almost normal, with cardinals chatting on the sidelines waiting their turn to say goodbye.

Benedict said he would pray for the cardinals in coming days as they discuss the issues facing the church, the qualities needed in a new pope and prepare to enter into the secret conclave to elect him.

"Among you is also the future pope, whom I today promise my unconditional reverence and obedience," Benedict said in his final audience.

Benedict's decision to live at the Vatican in retirement, be called "emeritus pope" and "Your Holiness" and to wear the white cassock associated with the papacy has deepened concerns about the shadow he might cast over the next papacy.

But Benedict has tried to address those worries over the past two weeks, saying that once retired he would be "hidden from the world" and living a life of prayer.

In his final speech in St. Peter's Square on Wednesday, he said he wasn't returning to private life exactly, but rather to a new form of service to the church through prayer.

Shortly before 5 p.m. Thursday, Benedict will leave the palace for the last time as pontiff, head to the helipad at the top of the hill in the Vatican gardens and fly to the papal retreat at Castel Gandolfo south of Rome.

There, at 8 p.m. sharp, Benedict will become the first pontiff in 600 years to resign. The doors of the palazzo will shut and the Swiss Guards will go off duty, their service protecting the head of the Catholic Church over - for now.

And on Monday, the cardinals are expected to begin meeting to set the date for the conclave.

Benedict's decision has been met for the most part with praise and understanding. Cardinals, Vatican officials and ordinary Catholics have rallied around him in acknowledgment of his frail state and the church's need for a strong leader.

But Sydney Cardinal George Pell has caused a stir by openly saying the resignation has been "slightly destabilizing" for the church.

In an interview with Australia's ABC radio, Pell noted that Benedict himself had acknowledged the shift in tradition; Benedict said Wednesday that he appreciated his decision was not only serious but "a novelty" for the church.