The Vatican described the 40-minute meeting at Castro's residence as informal and familial, with an exchange of books and discussion about big issues facing humanity, including Francis' recent encyclical on the environment and the global economic system.
Video of the encounter broadcast on Cuban state media showed the 89-year-old former president chatting animatedly with Francis and shaking the pope's hand, the pope standing in his white vestments and Castro sitting in a white button-down shirt and Adidas sweat top.
The meeting brought together the leader who shaped Cuba for the last half of the 20th century and Latin America's first pope, who many Cubans credit with opening a path to the future by mediating the warming diplomatic relations between their country and the United States. After his Cuba visit, the pope flies to Washington for his first ever trip to the U.S.
Since their historic deal, Presidents Barack Obama and Raul Castro have reopened embassies in each other's countries, held a personal meeting, had at least two phone calls and launched a process aimed at normalizing ties in fields ranging from trade to tourism to telecommunications.
Francis called on both Fidel and Raul Castro after celebrating Mass in Havana's main plaza on his first full day in Cuba. Believers and non-believers alike streamed into the square before dawn, and they erupted in cheers when the pope spun through the crowd in his open-sided popemobile. Francis wound his way slowly through the masses and stopping to kiss children held up to him.
While most Cubans are nominally Catholic, less than 10 percent practice their faith and Cuba is the least Catholic country in Latin America. The Vatican said 200,000 people attended Sunday's Mass, more than at similar celebrations in the same plaza by Pope Benedict XVI in 2012 and St. John Paul II in 1998.
In his homily delivered under the gaze of a metal portrait of revolutionary fighter Che Guevara, Francis urged Cubans to care for one another out of a sense of service, not ideology. He encouraged them to refrain from judging each other by "looking to one side or the other to see what our neighbor is doing or not doing."
"Whoever wishes to be great must serve others, not be served by others," he said. "Service is never ideological, for we do not serve ideas, we serve people."
"Being a Christian entails promoting the dignity of our brothers and sisters, fighting for it, living for it," Francis told the crowd.
Shortly after the Mass, Francis brought Fidel Castro three books including a volume of sermons by Fidel's former teacher, the Rev. Amando Llorente. Llorente taught at Colegio de Belen, a Jesuit high school where Fidel was a student. Llorente, a Spaniard, was forced out of the country after Castro's revolution and died in Miami in 2010.
Francis also brought two compact discs with Llorente's voice. Austen Ivereigh, author of "The Great Reformer: Francis and the Making of a Radical Pope," said he thinks the pontiff was using the Llorente books and recording to send a subtle message to Fidel, whose rule was marked by conflict with the Catholic Church and other groups.
Francis met afterward for an hour with Fidel's brother Raul, a declared atheist who has, perhaps jokingly, said he likes the pope so much he is thinking of returning to his Catholic roots. Francis thanked the 84-year-old leader for his pardon of thousands of petty criminals before his arrival. Castro presented the pontiff with a huge sculpture of the crucified Christ made of oars by the artist Kcho and a painting of the Virgin of Charity of Cobre, Cuba's patron saint.
The pontiff spoke off the cuff about the virtues of poverty at an evening vespers service in the San Cristobal Cathedral before a meeting with Cuban young people.
In an important aside, Francis ended Sunday's Mass with an appeal for Colombia's government and rebels, who have been holding peace talks in Havana for over two years, to put an end to South America's longest-running armed conflict.
"Please, we do not have the right to allow ourselves yet another failure on this path of peace and reconciliation," he said.
