Pope Leo spoke to crowds in St. Peter's Square on Sunday in his first public event since undergoing surgery for lung cancer five days ago, smiling from a window of the Vatican's Apostolic Palace as he spoke to pilgrims below in a firm, clear voice.

The 71-year-old pontiff had an operation on Tuesday to remove the lower lobe of his right lung, fuelling speculation about how his illness might change his 17-month papacy.

Leo came to the window in the palace unaided on Sunday and held his weekly noon-time appointment as usual. He thanked people across the world for their prayers and said he would slowly return to full duty as leader of the 1.4-billion member Church.

"Having overcome this delicate phase ... I will now gradually resume my activities, always with the help of God and your prayers," Leo said.

Pope Leo also urged pilgrims waiting below under cloudy skies to set aside any selfish ambitions and serve God "without compromise".

It is understood the Pontiff will travel to Latin America next month.