Pope Benedict XVI's announcement that he planned to resign Feb. 28 stunned and shocked religious leaders around the world.
Cardinal Keith O'Brien of St. Andrews and Edinburgh, Scotland, said he was "shocked and saddened" to hear of the pope's decision Monday.
"I know that his decision will have been considered most carefully and that it has come after much prayer and reflection," O'Brien said.
He offered prayers from the Scottish church for Pope Benedict "at this time of deterioration in his health as he recognizes his incapacity to adequately fulfill the ministry entrusted to him."
Archbishop Vincent Nichols of Westminster, England, president of the Bishops' Conference of England and Wales, said the pope's announcement "has shocked and surprised everyone."
"Yet, on reflection, I am sure that many will recognize it to be a decision of great courage and characteristic clarity of mind and action," Nichols said.
"The Holy Father recognizes the challenges facing the church and that 'strength of mind and body are necessary' for his tasks of governing the church and proclaiming the Gospel.
"I salute his courage and his decision," he added.
Anglican Archbishop Justin Welby of Canterbury said he learned of Pope Benedict's resignation with a "heavy heart but complete understanding." He offered thanks for the pope's priestly life "utterly dedicated in word and deed, in prayer and in costly service to following Christ."
"He has laid before us something of the meaning of the Petrine ministry of building up the people of God to full maturity," Welby said.
The Anglican archbishop credited the pope for his "witness to the universal scope of the Gospel and a messenger of hope at a time when Christian faith is being called into question." He cited Pope Benedict's teaching and writing for bringing a "remarkable and creative theological mind to bear on the issues of the day."
"We who belong to other Christian families gladly acknowledge the importance of this witness and join with our Roman Catholic brothers and sisters in thanking God for the inspiration and challenge of Pope Benedict's ministry," Welby added.
In Turkey, Msgr. Louis Pelatre, apostolic vicar of Istanbul, expressed surprise at Pope Benedict's the decision, telling Catholic News Service "no one expected this, even those very close to him. But we pray and go forward."
"It was his personal decision. No one can influence him. We are no longer in a world where one can stay in the same position if he no longer feels he is no longer capable of fulfilling his duties. He was very tired. We know that and we saw that," Pelatre said.
[Contributing to this story were Simon Caldwell in Manchester, England, and James Martone in Istanbul, Turkey.]