Pope Benedict XVI's decision to resign as head of the Catholic Church has shocked political leaders, religious figures and commentators alike.
Political reaction was led by Prime Minister David Cameron, who said: "I send my best wishes to Pope Benedict following his announcement. He has worked tirelessly to strengthen Britain's relations with the Holy See.
"His visit to Britain in 2010 is remembered with great respect and affection. He will be missed as a spiritual leader to millions."
Conservative MP Mark Pritchard, who has met Pope Benedict several times, said he would be a "sad loss" to the Catholic Church, while former MP Ann Widdecombe added: "He's given the church stability. He was very much an authority figure and he was very, very trusted by church."
Diarmaid MacCulloch, professor of the history of the church at Oxford University, said the announcement had taken many people by surprise.
"What's very noticeable is the sheer disarray around the statement," he said. "Clearly no one in the upper hierarchy in this country knew anything about it.
"Normally these things are very well prepared but this hasn't been. It may be the result of an elderly man reflecting in private and coming to a sudden decision."
Robin Gill, professor of applied theology at the University of Kent, described Pope Benedict as a "very intelligent, very astute man", who had managed to "hold Catholics together".
"I think he knew that if he continued, he would be unable to function properly," he said. "He saw his predecessor become increasingly frail and I think he has made a very wise decision."
Meanwhile, Madeleine Teahan, associate editor of the Catholic Herald, who was only 17 when Pope Benedict was elected, said: "Young people were so inspired and so encouraged by him."
Elsewhere, Monsignor Andrew Faley, of the Catholic Bishops' Conference, told Sky News: "He is extremely popular and is a Pope who will be remembered by ordinary people, as well as by political leaders, as a man who appreciated the humanity of everyone he met.
"He is someone who gives you his complete attention when you meet him and it's that sense of humanity and openness that he gave to everyone that he will be remembered for."
Father Christopher Jamison, a Benedictine monk, told Sky News: "My reaction is one of great shock and surprise.
"He's reached out very strongly to non-believers and fully recognises that people today won't necessarily join the Church.
"On his visit to the UK in 2010, people were surprised to discover that he wasn't a terrible ogre of a man but someone of great wisdom."
John Arnold, Auxillary Bishop of the Diocese of Westminster, recalled the Pope's trip to Britain with fondness.
"Those were four extraordinary days," he said. "He came as a friend and he came to give us a very important message about the Church in our country and our society."
The Most Reverend Justin Welby, the new Archbishop of Canterbury, added: "It wa with a heavy heart but complete understanding that we learned of Pope Benedict's declaration of his decision to lay down the burden of ministry as Bishop of Rome, an office which he has held with great dignity, insight and courage."
Further afield, the German government said it was "moved and touched" by the surprise resignation of Pope Benedict XVI, who was born in the country.
Spokesman Steffen Seiber said: "The German government has the highest respect for the Holy Father, for what he has done, for his contributions over the course of his life to the Catholic Church.
"He has been at the head of the Catholic Church for nearly eight years. He has left a very personal signature as a thinker and also as a shepherd."
In the Philippines, President Benigno Aquino said the country was grateful for the "many prayers and comforting words Pope Benedict XVI dedicated to Filipinos in times of calamity and challenge".
"All peoples and nations of goodwill are filled with great regret," his spokesman said.
Israeli Prime Minister Benjamin Netanyahu hailed the Pope's "courage", while Rabbi Yona Metzger, the country's Ashkenazi Chief Rabbi, said he had helped to reduce anti-Semitism around the world.
Meanwhile, French President Francois Hollande said the Pope's decision to resign was "eminently respectable".
However, some groups have expressed disappointment that the Pontiff is stepping down.
Anne Doyle from BishopAccountability.org believes he could have done more to help victims of child abuse.
"Joseph Ratzinger leaves the papacy having failed to achieve what should have been his job: to rectify the incalculable harm done to the hundreds of thousands of children sexually abused by Catholic priests. He leaves hundreds of culpable bishops in power and a culture of secrecy intact," she said.