Pope Benedict XVI formally resigns to become 'pilgrim'
Pope Benedict XVI has officially resigned, saying that he now "will simply be a pilgrim" starting his last journey on earth.
The pontiff, aged 85, was earlier flown by helicopter from the Vatican to his retreat at Castel Gandolfo, near Rome.
The college of cardinals, headed by Cardinal Tarcisio Bertone, will run the Roman Catholic Church until a new pope is elected.
Benedict vowed "unconditional obedience and reverence" to his successor.
He stepped down after nearly eight years in office - the first pontiff to do so in 600 years.
In normal times, popes make their final exit from the Vatican in a coffin and - after a period of mourning - are laid to rest in the crypt of St Peter's Basilica as the great bells of the first church of Christendom boom out.
But not Benedict XVI.
His nearly eight-year-long pontificate has ended not in death, but with a remarkable resignation.
Benedict was clearly relieved that the world has now accepted the fact that his physical health has declined to a point where he cannot continue to carry on the heavy responsibilities of leading his Church.
Pope Benedict's last words on his final day in office - as he greeted one-by-one the cardinals - were a plea to work together in harmony.
Benedict, a classical music lover, urged them to seek to play in harmony, just like an orchestra - for the future good of the Catholic Church.
Benedict officially ceased to be the Pope at 20:00 local time (19:00 GMT).
The resignation was marked by the papal Swiss Guards stepping down from their posts at Castel Gandolfo to return to the Vatican. The protection of Benedict was taken over by Vatican police.
In his retirement, Benedict will wear a simple white cassock rather than his papal clothes, and swap his famous red shoes - the colour is symbolic of the blood of the early Christian martyrs - for brown.
His "Fisherman's Ring", the special signet ring which contains the Pope's name and is impressed to validate certain official documents, is expected to be destroyed along with the lead seal of the pontificate.
The German pontiff, who was born Joseph Ratzinger, will continue to be known as Benedict XVI, with the new title of "pope emeritus".
The long-time theologian is expected eventually to retire to a monastery on a hill inside Vatican City, with officials saying he will not be able intervene publicly in the papacy of his successor, though he may offer advice.
The conclave of 115 cardinals is expected to meet at the Vatican on Monday morning to start planning the election of the next pope.Joy and sadness
Earlier on Thursday, bells of St Peter's rang across the Vatican as Benedict boarded the helicopter for a short flight to Castel Gandolfo.
Before that, the pontiff was greeted for the last time by top officials in the Curia - the administrative body that runs the Holy See.
Benedict then appeared at a window overlooking the public square in Castel Gandolfo to bless a cheering crowd.
"Thank you very much for your friendship," Benedict said.
"I will simply be a pilgrim who is starting the last phase of his pilgrimage on this earth.
"Let's go forward with God for the good of the Church and the world."
Some in the crowd were in tears listening to what could be Benedict's final public words.
Castel Gandolfo retreat
- The traditional lakeside summer home of the popes is a picturesque "castle town" in the Alban hills, 15 miles (24km) south-east of Rome
- The papal palace dates back to the 17th Century and its gardens occupy the site of a residence of the Roman Emperor Domitian
- Benedict XVI is expected to spend two months resting in the residence as his successor at the Vatican is chosen and installed
- He will have a staff of two secretaries and four women helpers, drawn from a Catholic lay organisation
- Vatican police officers will guard the former pope, who lost his Swiss Guards when his resignation took effect
"What a joy to see him, but how sad to think it is for the last time," local resident Giuseppina was quoted as saying by the AFP news agency.
In a final posting before his @Pontifex account was suspended and all its entries archived, Benedict tweeted: "Thank you for your love and support. May you always experience the joy that comes from putting Christ at the centre of your lives."
The Vatican now enters the Sede Vacante - or period of transition between two pontificates.
Benedict's successor must focus on reforming the Vatican bureaucracy which has often been overly hesitant to react to the various crises which have arisen during Benedict's papacy, the BBC's David Willey reports from the Vatican.Beset by scandals
On Thursday morning, the Pope received the cardinals at the Vatican's Clementine Hall, warmly embracing Cardinal Angelo Sodano, who passed on best wishes on behalf of those gathered.
"Among you there is also the future pope to whom I promise my unconditional obedience and reverence," the pontiff said.
"The Church is a living being," he added, but it "also remains always the same".
In his public farewell speech on Wednesday, Benedict hinted at Vatican infighting.
His decision to resign has been openly criticised by Australia's top Catholic, Cardinal George Pell, who questioned his leadership skills.
The Church has been beset by scandals over sexual abuse by priests and leaked confidential documents revealing internal corruption and feuding.
An estimated 150,000 people packed into St Peter's Square on Wednesday to hear Benedict speak in his last address there.