The Pontiff is expected to highlight continuing inequality and the need for peace during his tour on the continent.
It was a loud and joyful crowd that welcomed Pope Francis as his plane touched down at Kenya's Jomo Kenyatta International Airport for the start of his inaugural African tour.
The head of the Catholic Church is on a six-day trip to Africa, with plans to also travel to Uganda and the Central African Republic, despite safety fears.
He will only visit the capital cities.
All three nations have experienced sectarian violence, and, with the recent attacks in France, Mali, Tunisia and Egypt, the Pope's visit comes at a turbulent time.
Shortly after his arrival, he travelled to President Uhuru Kenyatta's official residence, the State House, where he planted a tree on the grounds.
President Kenyatta thanked him for choosing to stop in Kenya.
"Indeed, Your Holiness, we are honoured, humbled and privileged that you have chosen to begin your apostolic voyage to Africa right here in Kenya, and we take this opportunity to welcome you to Kenya as we also welcome you to Africa."
Speaking shortly afterward, Pope Francis has called for unity and understanding among Kenyans, saying it is the only way to achieve peace.
"Experience shows that violence, conflict and terrorism feed on fear, mistrust and the despair born of poverty and frustration. Ultimately, the struggle against these enemies of peace and prosperity must be carried on by men and women who fearlessly believe in, and bear honest witness to, the great spiritual and political values which inspired the birth of the nation."
Both Christianity and Islam are forecast to increase their substantial presences in sub-Saharan Africa.
While Christianity is expected to retain its majority status, the Muslim population is predicted to grow at a faster rate.
A study by Georgetown University in the United States estimates the number of Catholics in Africa could more than double by 2050, to almost half a billion.
With around one in three Kenyans now baptised Catholics, a public Mass (thur) was expected to draw a huge crowd.
Many Kenyans, like this woman, have expressed happiness at the Pope's appearance.
"We are so excited to have him here, and, personally, I'm very, very happy, because I've been praying for this moment. And I'm looking forward to the coming days when we will have time to be with Pope Francis."
Thousands of police officers have been stationed around Nairobi for his visit, with the same expected at his next stop, the Ugandan capital Kampala.
Kenya has suffered numerous attacks from the militant group al Shabab in recent years, including a mass shooting at a university in Garissa in April that left 148 people dead.
Almost 70 others were killed in 2013 when gunmen took over a Nairobi shopping mall.
But while divides along religious lines are expected to be a big topic of discussion, the Pontiff also expressed his concern over continuing environmental degradation across Africa.
"We have a responsibility to pass on the beauty of nature in its entirety to the future generations and an obligation to exercise a just stewardship of the gifts we have received. These values are deeply rooted in the African soul. In a world which continues to exploit, rather than protect, our common home, they must inspire the efforts of national leaders to promote responsible models of economic development."
Pope Francis is due to give a major statement on the environment ahead of the global climate-change summit in Paris next week.
But in Nairobi, he kept his words brief, saying simply in Swahili "God bless Kenya".
