* Benedict to live in papal summer palace temporarily
* Abdication takes place at 1900 GMT
* Pope urges Church to remain united
* Cardinals plan election once papacy officially vacant (Adds Pope leaving Vatican)
Pope Benedict left the Vatican on Thursday after pledging unconditional obedience to whoever succeeds him to guide the Roman Catholic Church at one of the most crisis-ridden periods in its 2,000-year history.
The first pope in six centuries to step down, Benedict flew off in a white Italian air force helicopter across Rome's red-tiled roofs to the papal summer villa south of the capital where he will take up temporary residence.
"I will continue to be close to you in prayer, especially in the next few days, so that you are fully accepting of the action of the Holy Spirit in the election of the new pope," he told cardinals in an emotional farewell at the Vatican's frescoed Sala Clementina.
"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.
The pledge, made ahead of the closed-doors conclave where cardinals will elect his successor, was significant because for the first time in history, there will be a reigning pope and a former pope living side by side in the Vatican.
Benedict appeared to be sending a strong message to the top echelons of the Church as well as the faithful to remain united behind his successor, whoever he is.
Some Church scholars worry that if the next pope undoes some of Benedict's policies while his predecessor is still alive, Benedict could act as a lightning rod for conservatives and polarise the 1.2 billion-member Church.
Pope Benedict will spend the first few months of his retirement in the papal summer residence, Castel Gandolfo, a complex of villas boasting lush gardens, a farm and stunning views over Lake Albano in the volcanic crater below the town.
Locals set up banners and balloons in the square outside the Papal Palace on Thursday to welcome the pontiff.
At 8 p.m. (1900 GMT) the papacy will be officially vacant and two Swiss Guards that ceremonially watch over the summer villa will march away and not return until the new pope takes possession of the hilltop residence.
Benedict will stay until April when renovations are completed on a convent in the Vatican that will be his new home.
With the election of the next pope taking place in the wake of sexual abuse scandals, leaks of his private papers by his butler, falling membership and demands for a greater role for women, many in the Church believe it would benefit from a fresh face from a non-European country.
A number of cardinals from the developing world, including Ghanaian Peter Turkson and Antonio Tagle of the Philippines are two names often mentioned as leading candidates from the developing world who listen more.
"At the past two conclaves, the cardinals elected the smartest man in the room. Now, it may be time to choose a man who will listen to all the other smart people in the Church," said Father Tom Resse, a historian and senior fellow at the Woodstock Theological Center at Georgetown University.
Benedict, wearing the white papal cassock and red cape he will shed after his resignation becomes official, urged the Church to strive to be "deeply united".
A lover of classical music, he compared the Church hierarchy to an orchestra with many instruments which should always seek to be harmonious.
"Let us remain united, dear brothers," said Benedict, who alluded to the scandals and reports of infighting among his closest aides.
"In these past eight years we have lived with faith beautiful moments of radiant light in the path of the Church as well as moments when some clouds darkened the sky," he said.
The pope said he had "tried to serve Christ and his Church with deep and total love".
NEW POPE FOR EASTER
Once the chair of St Peter is vacant, cardinals who have assembled from around the world will begin planning the conclave that will elect his successor.
One of the first questions facing these "princes of the Church" is when the 115 cardinal electors should enter the Sistine Chapel for the voting. They will hold a first meeting on Friday but a decision may not come until next week.
The Vatican seems to be aiming for an election by mid-March so the new pope can be installed in office before Palm Sunday on March 24 and lead the Holy Week services that culminate in Easter on the following Sunday.
In the meantime, the cardinals will hold daily consultations at the Vatican at which they discuss issues facing the Church, get to know each other better and size up potential candidates for the 2,000-year-old post of pope.
There are no official candidates, no open campaigning and no clear front runner for the job. Cardinals tipped as favourites by Vatican-watchers include Turkson, Tagle, Brazil's Odilo Scherer, Canadian Marc Ouellet, Italy's Angelo Scola and Timothy Dolan of the United States.
Benedict, a bookish man who did not seek the papacy and did not enjoy being in the global spotlight, proved an energetic teacher of Catholic doctrine but a poor manager of the Curia, the Vatican bureaucracy that became mired in scandal.
He leaves his successor a top secret report on rivalries and scandals within the Curia, prompted by leaks of internal files last year that documented the problems hidden behind the Vatican's thick walls and the Church's traditional secrecy.