Riding through the streets in his open-sided popemobile, the pontiff waved to cheering, sceaming, singing, flag-waving crowds as he made his way up the Benjamin Franklin Parkway and reached the altar at the foot of the broad steps of the Philadelphia Museum of Art.
It was the final stop on his itinerary before the 78-year-old pontiff was to return to Rome.
En route to the Mass, Francis stepped off his popemobile to see an art installation with particular meaning for him: Inside the grotto outside Philadelphia's cathedral were 30,000 white knotted ribbons, each representing a personal hardship or societal challenge.
The exhibit was inspired by one of Francis' favorite paintings, "Mary, Undoer of Knots," showing Mary untangling a long ribbon symbolizing life's difficulties.
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The painting hangs in a church in Germany, where then-Rev Jorge Mario Bergoglio saw it while studying in the mid-1980s.
He assured them that he believes them and that bishops will be called to account for what they did.
"I pledge to you that we will follow the path of truth wherever it may lead," Francis said in Spanish. "Clergy and bishops will be held accountable when they abuse or fail to protect children."
Then, he went into a meeting with American bishops in town for a Catholic festival on the family and told them the same thing face-to-face.
Along the way, he drew large and adoring crowds, met with President Barack Obama, visited ground zero and addressed Congress and the United Nations, calling for urgent action on climate change and poverty.
The meeting today with victims of sexual abuse was the second one Francis has held. He received some at the Vatican in July 2014.