The pontiff, in Krakow to headline an international Catholic youth extravaganza, travelled to the southern city of Czestochowa to pray before the legendary Black Madonna and say mass.
Security was tight at the hill-top shrine following a series of attacks in Europe, with a highly visible police and army presence on the roads leading into the city, where hundreds of thousands of pilgrims were gathered.
Francis, 79, hopped in an open-topped pope mobile for a tour through the cheering crowds.
Teenagers still wrapped in sleeping bags, elderly couples perched on folding chairs outside tents and families with Francis-themed balloons were serenaded by the Argentine pope's favourite tango music before his arrival.
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Ambulances and fire-engines were on standby with Polish officials taking no chances with security following the jihadist murder of a priest in a French church on the eve of the five-day trip to celebrate the 2016 World Youth Day.
He was set to lead a mass to mark the 1050th anniversary of Poland's Catholic faith at the monastery.
The Black Madonna has been venerated since 1711, when the bubonic plague decimated Warsaw's population but stopped dead when it reached the Jasna Gora monastery in the south of the country.
Since then the figure, which legend has it was painted by Saint Luke the apostle, has been hailed as miraculous.
Francis will return to Krakow after the mass, where he will be presented with the keys to the city by the mayor, before jumping on to a tram which will whisk him to the sprawling Blonia meadow for his first big WYD event.
Some 15 disabled youngsters and their families will take the tram with the pontiff to the welcome ceremony, where hundreds of thousands of people will have spent the day being entertained by singers and dancers.