Pope Francis has praised his predecessor Pope Benedict XVI as courageous and humble on the first anniversary of his remarkable announcement that he would retire.

In a tweet Tuesday, Francis wrote: "Today I ask you to join me in prayer for His Holiness Benedict XVI, a man of great courage and humility."

It was the latest, and surely most authoritative, in a long line of praises being showered on Benedict, who became the first pope in 700 years to retire when he stepped down last year.

His resignation has been seen by many in the Vatican as a great act of governance, enabling the reform-minded Francis to clean house in the Vatican and reboot a 2,000 year-old institution hobbled by sex abuse scandals and growing indifference in a secularized world.