Pope Francis celebrated Sunday Mass in Havana, warning Cubans of the dangers of ideology.
Francis preached before tens of thousands of worshippers who packed Havana's historic Revolutionary Square
"Christians are constantly called to set aside their own wishes and desires, their pursuit of power, and to look instead to those who are most vulnerable," he said. "All of us are asked, indeed urged, by Jesus to care for one another out of love...without looking to one side or the other to see what our neighbor is doing, or not doing."
Francis paid a visit to former Cuban leader Fidel Castro at his home in Havana. With Castro's family looking on, the two met in what the Vatican called "a very informal and friendly environment."
Pope Francis and Cuba's Fidel Castro shakes hands, in Havana, Cuba, Sept. 20, 2015.
They also exchanged books on religion as gifts.
Pope Francis later met Fidel's brother and current Cuban President Raul Castro, and was to hold evening prayers with a group of priests and speak to youths at a cultural center.
Francis and Vatican officials were the force behind months of secret talks between Havana and Washington in 2014. The talks climaxed with last year's announcements by Raul Castro and U.S. President Barack Obama to reestablish diplomatic ties for the first time in 53 years. They have since opened embassies in Washington and Havana.
Francis praised the détente between the two long-estranged neighbors as "an example of reconciliation for the entire world" that "fills us with hope."
From left, Panama's First Lady Lorena Castillo Garcia de Varela, Cuban President Raul Castro and Argentine President Cristina Fernandez de Kirchner wait for Pope Francis to arrive at Revolution Square in Havana, Sept. 20, 2015.
This is the third papal visit to Cuba since 1998, when Pope John Paul the Second came to the island, followed by Pope Benedict in 2012.
Francis will also travel to the Cuban cities of Hoguin and Santiago during his visit, with plans to hold Mass and meet with Catholic clergy in both cities before leaving for Washington.
His U.S. schedule includes a private meeting with President Obama, speaking to a joint meeting of Congress, and addressing the United Nations General Assembly in New York.
Pope Francis's visit marks a turning point for the Catholic Church in Cuba, which was marginalized for years under Castro's communist rule, but has slowly re-emerged as as powerful force on the Caribbean island.
Pope Francis holds his pastoral staff as he arrives to celebrate Mass at Revolution Plaza in Havana, Cuba, where a sculpture of revolutionary hero Ernesto "Che" Guevara and a Cuban flag decorate a nearby government building, Sept. 20, 2015.
Upon his arrival in Havana, Francis praised the detente between the two long-estranged neighbors as "an example of reconciliation for the entire world" that "fills us with hope."
'Sign of victory'
He called the thawing of relations between the two nations "a sign of the victory of the culture of encounter and dialogue."
In a 15-minute speech that quickly turned political, President Castro repeated calls for the U.S. to end its embargo of the island, which he called “cruel, immoral and illegal,” and for the return of Guantanamo Naval Station.
Washington imposed the embargo in 1962, and the current Republican-led Congress has ignored President Obama's calls to end it.
Faithful attend Mass celebrated by Pope Francis at Revolution Plaza in Havana, Cuba, Sept. 20, 2015.
The United States' U.N. ambassador in turn tweeted criticism of Cuban authorities over reported detentions in advance of Pope Francis' visit.
Ambassador Samantha Power's tweet said human rights activists "and even homeless reportedly detained before @Pontifex visit; disappointing business as usual for #Cuban govt."
Opposition groups have been reporting increased detentions of dissidents. Cuba's government has not commented on the claims.
The pontiff's U.S. schedule will include a private meeting with Obama, who will greet Francis when he arrives at Joint Base Andrews outside Washington; speeches before a joint meeting of the U.S. Congress and the United Nations in New York; and an appearance before a Vatican-sponsored World Meeting of Families in Philadelphia.
Some material for this report came from AP and AFP.