The pontiff told an estimated one million people at a service in the Banado Norte shanty-town, Paraguay, that the church should be a place of welcome for all.
"Welcoming those who do not think as we do, who do not have faith or who have lost it," he urged, "Welcoming the persecuted, the unemployed. Welcoming the different cultures, of which our Earth is so richly blessed. Welcoming sinners."
"How much pain can be soothed, how much despair can be allayed in a place where we feel at home," Francis added.
The pope said he had decided to visit a shanty-town to encourage the residents' faith despite the difficulties they encounter.
He said he wanted "to see your faces, your children, your elderly, and to hear about your experiences and everything you went through to be here, to have a dignified life and a roof over your heads, to endure the bad weather and the flooding of these last few weeks".
Francis urged people to "stir things up" before he left for his flight back to Rome.
But he added they had to "clean it up and fix the mess that you've made".