Pope Francis made a plea for traditional values, saying “the health of any society depends on the health of its families”.
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.
Crowds in the capital, Nairobi, waited in the rain at the University of Nairobi sports ground since the early hours of Thursday morning. More than one million were expected to attend the Mass.
Wearing a robe embroidered to look like beads worn by the Maasai, Pope Francis told them: “Our faith in God’s word calls us to support families in their mission in society, to accept children as a blessing for our world, and to defend the dignity of each man and woman, for all of us are brothers and sisters in the one human family.”
He also spoke about abortion and the need for a caring society: “We are also called to resist practices which foster arrogance in men, hurt or demean women, and threaten the life of the innocent unborn.”
And he appealed to young Kenyans “to shape a society which is ever more just, inclusive and respectful of human dignity”.
He said they “should reject everything that leads to prejudice and discrimination”.
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.
The Pope earlier said conflict and terrorism fed “on fear, mistrust, and the despair born of poverty and frustration”.
The pontiff has played down security fears about his trip, joking that he was “more worried about the mosquitoes”.
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.
Militant Islamists have carried out a series of attacks in Kenya – including the 2013 siege at Nairobi’s Westgate shopping centre, which left at least 67 dead, and the killing of about 150 people during an assault on the Garissa National University College in April this year.
Pope Francis is later expected to visit the headquarters of the UN Environment Programme, and he has already spoken of a “grave environmental crisis” facing the world, and said leaders needed to promote “responsible models of economic development”.
Speaking on Wednesday he also made a veiled reference to corruption by calling on leaders to work with integrity and transparency, says the BBC’s Joseph Odhiambo in Nairobi.
President Kenyatta has called on the Pope to pray that Kenya succeeds in its fight against corruption.
About 30% of Kenyans – including President Kenyatta – are baptised Catholics.
Source: BBC