Pope Benedict XVI on Thursday vowed "unconditional obedience" to his successor on his historic final day as leader of the world's 1.2 billion Catholics, when he will become the first pontiff to resign since the Middle Ages.

"Among you there is also the future pope to whom I promise my unconditional obedience and reverence," the pope said as he bade farewell to cardinals in the Vatican's ornate Clementine Hall.

"Let the Lord reveal the one he has chosen," said the 85-year-old pope, wearing an ermine-lined red stole over his white cassock.

"We have experienced, with faith, beautiful moments of radiant light together, as well as times with a few clouds in the sky," Benedict said, reprising a theme from his adieu to some 150,000 pilgrims in St Peter's Square on Wednesday.

The cardinals with their black cassocks and red sashes then took turns bidding farewell to the pontiff, kissing his gold papal signet ring according to time-honoured tradition.

Many doffed their berettas in a sign of deference.

Just hours remained before Benedict will make history as only the second pope to resign of his own free will in the Church's 2,000-year history.

The German pope stunned the globe when he announced on February 11 his surprise decision to step down.