Pope Francis, who is recovering after a serious case of pneumonia, made a surprise public appearance on Sunday as he mingled with crowds at the Vatican just two weeks after leaving hospital.

"A good Sunday to all. Thank you very much", the 88-year old Pope said as he was wheeled through Saint Peter's Square in his wheelchair after a mass dedicated to the sick.

Surrounded by a mass of smartphones and cameras, and attached to nasal breathing tubes, his voice was fragile but more audible than when he left the Gemelli hospital on March 23. His last public appearance was February 14.

Francis is, in theory, undergoing two months of convalescence following his life-threatening health scare, with strict rest, no public activity and no contact with crowds. (AFP)