The 79-year-old Francis, walking in his long robe with an incense holder in his hand, did not notice a step down from the platform and fell to the ground before the altar.
He braced his fall with his left hand and priests around him rushed to help, with Monsignor Guido Marini, an aide, helping him back to his feet.
The Mass proceeded as planned and the Pope delivered a long homily before tens of thousands of faithful gathered at the foot of the Jasna Gora monastery in the city of Czestochowa.
Asked if Francis had suffered any ill effects from the fall, a Vatican spokesman, Greg Burke, said “the Pope is fine”.
Czestochowa Archbishop Waclaw Depo said Francis fell because he had closed his eyes and appeared to miss a step.
“He is in good condition. He did not even complain at all. He never said a word.
“Also the homily showed that the Pope has strength and this strength he gets from the people,” he said.
Francis enjoys relatively good health, despite putting in long days of ceremonies, audiences, and meetings. In his youth, he had a section of one lung removed, and on occasion has sounded somewhat winded.
A few times in the past, Francis has missed a step or even fallen on stairs. Each time he has gotten up on his own or thanks to an aide lending a hand.
The Pope’s visit to Poland has been accompanied by several messages. Several days before he left the Vatican, his office admonished countries in Eastern Europe that have “artificially created fear of Muslims,” urging them to be more open to refugees and asylum seekers from places torn apart by conflict.
On his way to Krakow on Wednesday, the pope warned the world was “at war,” but he emphasised that he was referring to conflicts over resources: “I am not speaking of a war of religions. Religions don’t want war.”
In his homily on Thursday morning, he spoke again of the trappings of power and lack of modesty. He was scheduled to address the World Youth Day gathering later yesterday, and, today, to visit the Second World War-era death camp at Auschwitz-Birkenau.