Mr. Morales, a fierce critic of American corporate influence, wore a white shirt and a dark jacket bearing a picture of the Communist revolutionary Che Guevara on the left breast.

“For the first time, I feel like I have a pope: Pope Francis,” Mr. Morales said.

Francis has filled four consecutive days with appearances, but other than an environmental critique offered in Ecuador, the pope had hewed mostly to theological topics or broad themes like family, service and mission.

His appearance on Thursday night was at the Second World Meeting of Popular Movements, a congress of global activists working to mobilize and help the poor. Some people wore Che Guevara T-shirts while some indigenous women wore traditional black bowlers.

Francis drew cheers when he called on the activists and others to change the social order: “I would even say that the future of humanity is in great measure in your own hands, through your ability to organize and carry out creative alternatives, through your daily efforts to ensure the three Ls — labor, lodging, land.”

Francis repeated his condemnation of an economic system rooted in pursuit of money and profits, but in an aside he criticized “certain free-trade treaties” and “austerity, which always tightens the belt of workers and the poor” — a likely reference to Greece.