Pope Francis meets Fidel Castro after saying Mass in Havana

Pope Francis met with Fidel Castro on Sunday after urging thousands of Cubans who filled Havana's evocative Revolution Plaza to care for one another, and not judge each other, as he opened his visit to the country amid great hopes that the key role he played in bringing about detente with the U.S. will result in changes on the island.

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 heads to his first ever trip to the U.S.

An estimated 50 dissidents were detained and prevented from attending papal events, according to the dissident Cuban Commission for Human Rights and National Reconciliation.

Most were released after a few hours but the Patriotic Union of Cuba (UNPACU), the country's largest dissident group, said four of its activists remained in jail as of late Sunday. 

Earlier in the day, believers and non-believers alike streamed into the square before dawn for Francis' Mass, some of them wearing matching clothes to mark themselves as part of the various parishes and other groups gathered for the papal visit.

By 6 a.m., every spot in the Plaza available to the public was full, and the crowd erupted in cheers when history's first Latin American pope made his first drive through the crowd in his open-sided popemobile.

Francis didn't disappoint, winding his way slowly through the masses and stopping to kiss children held up to him.

The crowds endured hot, humid weather, but the sun stayed behind clouds as the assembled multitudes fanned themselves with anything available. Several children from parishes around Havana took their first communion from the Pope while a small army of priests and lay Catholics distributed communion among the crowd. Spirited Cuban music floated around the gathering, a local touch that many in the crowd appreciated.

“The music was great; it was really Cuban music and in our own language,” said Yaima Soto Mora, 14. The chorus, she noticed, were wearing caramel-colored robes topped with green, just like Cuba’s national tree, the royal palm.

While most Cubans are nominally Catholic, fewer than 10 percent practice their faith. The crowd was not as big as when St. John Paul II became the first pope to visit the island in 1998, but it drew a varied crowd of people, including many non-Catholics who wanted to hear Francis' message.

In his homily, Francis spoke in his native Spanish and urged Cubans to care for one another out of a sense of service, not ideology, and to refrain from judging one another by “looking to one side or the other to see what our neighbor is doing or not doing.”

“Being a Christian entails promoting the dignity of our brothers and sisters, fighting for it, living for it,” Francis told the crowd. “That is why 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.”

Several people compared his style favorably to that of his predecessor, Pope Benedict XVI, who retired in 2012. “With Pope Benedict half of the mass was in Latin so we could not understand it,” said Naomi Lopez Moran, 19, an engineering student who said she practices the religion of the Yoruba people of Nigeria. “I think that even the Catholics could not understand him at all. Pope Francis is so different. 

Maria Regla González, a 57-year-old teacher, said she appreciated Francis' message of reconciliation and unity for all Cubans.

“This is a crucial moment, and the pope's support for us is very important,” she said. “He made a call for unity, and that's what we want.”

The morning Mass kicked off a busy series of events for Francis, including a formal meeting with President Raul Castro. Francis will finish with an evening vespers service in the San Cristobal cathedral and meet with Cuban young people.

In an important aside, Francis ended the 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.

The appeal followed the historic call he issued to Presidents Barack Obama and Cuban President Raul Castro to end their half-century of estrangement that resulted in the restoration of diplomatic relations this summer. Since then, the two leaders 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.

Jose Rafael Velazquez, a 54-year-old worker, arrived with his wife at the plaza three hours before Mass began. He said he isn't religious, but came more out of curiosity to witness a historic event.

“We also are very hopeful for this visit, because the pope was key in the deal with the United States,” he said. “Ever since the announcement, there have been changes, and this visit gives me more hope that it'll get better.”

The Vatican has long opposed the U.S. trade embargo on the grounds that it hurts ordinary Cubans most, and is clearly hopeful that detente will eventually lead to a lifting of sanctions.

But only the U.S. Congress can remove the embargo. Francis will visit Congress next week at the start of the U.S. leg of his trip, but it's not known if he will raise the issue there.

Al Jazeera and The Associated Press. Additional reporting by Maria Duran in Havana.