The first pope from Latin America waved from his popemobile at a park in Guayaquil, Ecuador's largest city, where a jubilant crowd of more than 600,000 people welcomed him, according to official figures.
A sea of Catholics, many from neighbouring countries, held pictures of the pope or images of the Virgin Mary at Los Samanes park, as firefighters sprayed the crowd with water to keep them cool amid punishing heat.
Authorities had anticipated a crowd of more than one million people on the second day of the pope's trip to the region, which will also include stops in Bolivia and Paraguay.
The 78-year-old Argentine-born pontiff declared when he arrived in Ecuador on Sunday that the plight of the continent's myriad poor would be a dominant theme of his tour.
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"Progress and development must ensure a better future for all, paying special attention to our most vulnerable brothers and the most vulnerable minorities, which are the debt that Latin America still has," Francis said.
It is the first visit by a pontiff to Ecuador in three decades.
Rosa Elena Lata, 82, travelled 16 hours from southern Ecuador to attempt what she called the "heavenly miracle" of seeing the pope, "because seeing him will be like seeing Jesus."
Irma Guaita, 49, shielded her head from the sun with a piece of cardboard.
"The sun doesn't matter because God gave me a chance to see him," she said.
Before the mass, Francis, the first Jesuit pope, visited the Shrine of the Divine Mercy on the outskirts of town, where he blessed some 2,000 people, including a group of people with disabilities.
After the mass, he will have lunch with fellow Jesuits, among others, before returning to Quito for a meeting with President Rafael Correa and a visit to the Metropolitan Cathedral, in the historic heart of the capital.