The pontiff used the occasion to tell them their families are the bedrock of society.
In his homily, Francis praised families as the nucleus of society, calling them "the nearest hospital, the first school for the young, the best home for the elderly".
With temperatures reaching 30C (86F) in the Pacific port city of Guayaquil, fire-fighters sprayed water from hoses to provide relief to the crowd. Many had spent the previous night there to get the best spot.
The pontiff arrived in his popemobile to cheers of "Francisco! Francisco!" before he allowed pilgrims to take selfies with him.
The mass was the first big event of his nine day tour of South America.
It's expected to be his most gruelling tour yet with 22 speeches to be delivered, seven flights and fifteen thousand miles covered.