Pope Francis made a plea for traditional values, saying "the health of any society depends on the health of its families," BBC News reports.
The Pope earlier urged Kenyans to work for peace and reconciliation on his first trip as pontiff to Africa, amid a rise in militant violence.
He arrived in Kenya on Wednesday, the first stop on a three-nation tour.
Ahead of the Mass, Francis had been meeting with religious leaders, who he said should be "prophets of peace" in a violent and hate-driven world.
Read alsoPoroshenko: Pope accepts invitation to visit Ukraine A leading Muslim cleric in Kenya welcomed the visit, saying it gave hope to the "downtrodden in the slums."
Pope Francis's five-day visit will also see him go to Uganda and Central African Republic, which has been hit by Christian-Muslim conflict.
Kenya's government has said that up to 10,000 police officers may be deployed during the visit.
About 30% of Kenyans – including President Kenyatta – are baptized Catholics.
