VATICAN CITY: 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.

"Let us remain united, dear brothers," he said, in the final moments of an eight-year pontificate often overshadowed by infighting at the Vatican and divisions between reformers and traditionalists in the Catholic Church.

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 of the 144 cardinals doffed their berettas in a sign of deference and offered a few parting words to the pope.

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.

At 1600 GMT he will board a helicopter and see the Vatican from the sky one last time as pope before he goes into retirement.

Initially he will live in the papal residence of Castel Gandolfo near Rome and within a couple of months he is expected to move into a former convent in the Vatican grounds.

A final tweet will be published on his Twitter account @pontifex before it is suspended until the new pope decides whether he too wants to be a microblogger.

A bell in Rome city hall on the Capitol -- the Italian capital is the papacy's diocese -- will ring to mark the moment the pope leaves the city.

The German pope stunned the globe when he announced on February 11 his decision to step down, saying he no longer had the "strength of mind and body" required by a fast-changing world.

The news has captured massive media attention, with the Vatican saying Thursday that 3,641 journalists from 61 countries will cover the upcoming conclave -- on top of the regular Vatican press corps.

After a brief greeting to the residents of Castel Gandolfo -- effectively Benedict's last remarks before he retreats out of the public eye -- the pope will retire for prayer in a private chapel.

At 1900 GMT, his powers will formally expire.

The moment Benedict is no longer pope will be marked by Swiss Guards -- the papacy's own military since the 15th century -- closing the gates of Castel Gandolfo, after which they will change out of their colourful striped uniforms and return to Rome, their papal protection duties completed.

In the Vatican, staff will apply seals to the papal apartments in the Apostolic Palace and to the lift that goes directly to them. The seals will be kept in place until a new pope is elected.

The ex-pontiff will formally carry the new title of "Roman Pontiff Emeritus" or "pope emeritus" for short, although he will still be addressed as "Your Holiness Benedict XVI".

The only other pope who resigned by choice was Celestine V, a humble hermit who stepped down in 1294 after just a few months in office out of disgust with Vatican corruption and intrigue.

Once Benedict takes up permanent residence on a hill within the Vatican walls, the Church will find itself in the unprecedented situation of having a pope and his predecessor living within a stone's throw of each other.

Commenting on the new arrangement, Vatican spokesman Federico Lombardi said that Benedict "has no intention of interfering in the positions, decisions or activities of his successor".

Benedict has said he will live "hidden from the world" but the Vatican said he could provide "spiritual guidance" to the next pope.

Vatican analysts have suggested his sudden exit could set a precedent for ageing popes in the future, and many ordinary Catholics say a more youthful, pastoral figure could breathe new life into a Church struggling on many levels.

But Australia's top Catholic, Cardinal George Pell, said he was concerned at the implications.

"People who, for example, might disagree with a future pope, will mount a campaign to get him to resign," Pell told Australian television.

Pell, who will help choose the new pope as one of 115 "cardinal electors" expected at next month's conclave, also criticised the outgoing pope, saying the departing pontiff was a "brilliant teacher" but "government wasn't his strong point".

From Catholic reformers calling for women clergy and for an end to priestly celibacy, to growing secularism in the West and ongoing scandals over sexual abuses by paedophile priests going back decades, the next pope will have a tough agenda.

Meanwhile the suspense was building up in Castel Gandolfo -- a tiny hilltop town which for centuries has hosted popes escaping the sweltering Roman summer.

"It's a very emotional day," said Patrizia Gasperini, 40, who works in a gift shop next to the wooden doors that will close on Benedict's papacy.

Gasperini, who named her eight-year-old daughter Benedetta in his honour, said: "We've been privileged to see a different, more humane side to him over the years, and grown to love him."