In Washington, President Barack Obama offered "appreciation and prayers" on behalf of all Americans, saying he and his wife Michelle had warm memories from a 2009 meeting.
British Prime Minister David Cameron said Benedict would be "missed as a spiritual leader to millions" who had "worked tirelessly to strengthen Britain's relations with the Holy See."
And one of the most glowing tributes came from Brazil's Catholic bishops, who hailed the outgoing pontiff's "humility and greatness."
Closer to the Vatican, Italian President Giorgio Napolitano saluted the pontiff's "courage" over the decision to quit, making him the first pope in more than 700 years to step down, and after less than eight years in post.
Australian Prime Minister Julia Gillard focused on the difficulties of the pope's decision, and she praised him for nevertheless taking such a step.
"On his election, Joseph Ratzinger said he wished to be 'a simple humble worker in the vineyard of the Lord' and in his resignation that humility has been amply demonstrated," Gillard was quoted by Australian Associated Press (AAP) news agency as saying.
Joseph Ratzinger was the pope's name before he was made pontiff.
Bishop Marcelo Sanchez Sorondo, chancellor of the Vatican's Pontifical Academy of Sciences, said the pope's announcement caused "dismay and great sadness (in the Holy See) because he is a great pope."
From other world religions, Israel's Ashkenazi Chief Rabbi Yona Metzger told AFP that Benedict had improved ties between Judaism and Christianity that helped reduce anti-Semitism around the world.
Benedict's papacy "elevated Catholic-Jewish relations onto an unprecedented level," World Jewish Congress president Ronald Lauder said in a statement.
"No pope before him visited as many synagogues."
Justin Welby, head of the 85-million-strong worldwide Anglican communion as the new archbishop of Canterbury, said Benedict held his office with "great dignity, insight and courage."
During his historic 2010 visit to Britain, the pope proved "a witness to the universal scope of the gospel and a messenger of hope at a time when Christian faith is being called into question," said Welby.
In the mainly Catholic Philippines, President Benigno Aquino's office highlighted the sympathy the pontiff expressed for Filipinos when deadly storms and other disasters hit the country.
And bishops in Spain -- a country the pope visited three times -- said they felt "orphaned" by the resignation, having felt "secure and enlightened by his rich teaching and his paternal closeness."
-- A 'more quiet' style --
In Uganda, Kampala Archdiocese chancellor Joseph Kazibwe Ntuwa said Benedict had exhibited a different, "more quiet" style of leadership than his predecessor John Paul II.
Benedict's leadership on core issues such as abortion and contraception had met with approval from Africa's traditionally more conservative Catholics, he added.
The secretary general of the Mexican Episcopal Conference, Bishop Eugenio Lira Rugarcia, remarked that the pope appeared diminished physically when he visited Mexico in March, but praised his leadership.
"Benedict XVI has been a completely open and transparent man, and this was a very honest decision," Lira Rugarcia told a news conference.
"The papacy of Benedict XVI has been very effective. His hand has not trembled in order to act firmly. He has taken very clear decisions."
But such remarks were met with scorn by groups representing victims of child abuse in Catholic-run institutions. They welcomed the resignation.
"This pope had a great opportunity to finally address the decades of abuse in the church, but at the end of the day he did nothing but promise everything and in the end, he ultimately delivered nothing," said John Kelly, of the Irish Survivors of Child Abuse support group.
In Australia, the Survivors Network of those Abused by Priests issued a statement saying the pontiff had done little to stop "the reign of terror of child rapist priests," according to AAP.
And the US branch of SNAP said the pope "still has two weeks" to take action against child sex abuse by church staff.
"Before he steps down, we hope he will show true leadership and compassion and take tangible action to safeguard vulnerable children," the group said.
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