HAVANA - Pope Francis spoke to Cubans Sunday about the need to serve others and later met privately with revolutionary leader Fidel Castro.

The meeting with Castro took place at the former leader's home hours after the pope celebrated a Mass with tens of thousands of Cubans in the Plaza of the Revolution, Vatican spokesman Federico Lombardi said.

They exchanged books and apparently spoke for about 40 minutes, Lombardi said. Castro gave the pope an interview book titled "Fidel and Religion."

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The pope gave Castro a book written by a Jesuit who taught Castro when he was in a Catholic school. Fidel was educated by Jesuits at an elite Catholic high school in Havana, as was his brother Raúl, Cuba's current president. The pope also gave Fidel a copy of his encyclical from June.

The two also spoke about some of the great issues of the day, Lombardi said.

During his homily earlier Sunday, Francis spoke of the need for service, but cautioned how people should perform it.

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"There is a kind of 'service' which truly 'serves,' yet we need to be careful not to be tempted by another kind of service, a 'service' which is 'self-serving,' " he said before a crowd of tens of thousands of people during his first visit to the Communist nation.

In a gentle warning, the pope added, "There is a way to go about serving, which is interested in only helping 'my people,' 'our people.' This service always leaves 'your people' outside, and gives rise to a process of exclusion."

Longtime Vatican analyst John Allen said those comments and others during Mass represented what could be construed as the pope's first-ever rebuke of the Castro government.

"This clearly was a kind of thinly veiled and admittedly fairly gentle critique of the Cuban regime and Cuban social revolution," said Allen, who was at the Mass.

Referring to the pope's comments that service should never be ideological, Allen said, "For Cubans raised on decades of party indoctrination, they will clearly hear that as a kind of mild rebuke.

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"Up until the Mass today, this papal stop has been good news for the Castro regime," Allen said. "I think this is probably the first time when the Castro brothers will come away thinking they have something to chew on."

Cuban leader Raúl Castro was among those in attendance during Mass, as was Argentine President Cristina Elisabet Fernández de Kirchner.

After arriving about 8:15 a.m. and greeting the crowds in his iconic, bubblelike popemobile, Pope Francis offered his homily on the importance of humility and selflessness, retelling the biblical story of Jesus asking his apostles what they had been discussing during their travels. The disciples didn't respond, ashamed to admit they had been arguing about who among them was the most important.

"The history of humanity has been marked by the answer we give to this question," Francis said in a Vatican translation of his prepared remarks. "Far from any kind of elitism, the horizon to which Jesus points us is not for those few privileged souls capable of attaining the heights of knowledge or different levels of spirituality."

Pope Francis used that message to call on Christians around the world to serve others, particularly the poor and vulnerable.

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Cuban security officials removed at least three people who were yelling and appeared to be distributing fliers before Pope Francis' Mass. Officials collected the pamphlets the three had been distributing, and the protesters' message was not immediately clear.

Speaking to the people of Cuba, the pope also said in the official translation, "God's holy and faithful people in Cuba is a people with a taste for parties, for friendship, for beautiful things. It is a people which marches with songs of praise. It is a people which has its wounds, like every other people, yet knows how to stand up with open arms, to keep walking in hope, because it has a vocation of grandeur.

"Today I ask you to care for this vocation of yours, to care for these gifts which God has given you, but above all I invite you to care for and be at the service of the frailty of your brothers and sisters. Do not neglect them for plans which can be seductive, but are unconcerned about the face of the person beside you." Many in attendance said they spent the night in the plaza, and traveled from throughout the country to be here.

"It's a gift from God and a blessing for the entire country," Barbara Rodriguez Acosta, 61, said in Spanish. She said she took a nine-hour bus ride from the interior of Cuba, arrived in the plaza around 12:30 a.m. and spent the night there in anticipation of the pope's Mass.

Acosta said before the mass that she had not eaten since 6 p.m. Saturday, when the bus stopped for a food break during the trip. But that did not bother her -- she was so excited to see the pope.

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"It's an opportunity that you can't miss," she said before the pope's Mass. "I don't care if it rains or there is brutally hot sun -- I'm staying."

Olga Maria Pozo, 43, who waited in the plaza from 10 p.m. Saturday night, said the pope's visit and Mass were especially important because of the role he has played in helping Cuba and the U.S. re-establish relations.

"It means a lot now because he is mediating between Cuba and United States," she said in Spanish. "It's a blessing for all of the Cuban people." The crowd also included nonbelievers, many drawn by the pope's charisma.

"He's a very revolutionary pope, very advanced," Maria Rodriguez, 60, a non-Catholic, said in Spanish.

Artist and translator May Antonio Perez Garcia, 45, came to the Mass despite being an atheist.

"I had to come because it's a historic happening," he said in Spanish. "You had to experience it in person."

It was momentous, he said, in part because it is the third time the Communist-run government has allowed a pope to speak here -- Pope Benedict XVI and Pope John Paul II also visited.

"The government of Cuba is finally showing signs of intelligence and recognizing it has committed many errors, as much in the area of religion as in the politics between the United States and Cuba," he said.

Allowing three popes to speak here "in the past would have been impossible," he said. "That the government has permitted three popes to give Masses in the plaza of Jose Marti, the principal place of speeches of the Communist ideology -- to share this space with talks totally different is a symptom that the Castro dynasty simply has recognized how mistaken they have been."

The Mass, which began shortly after 9 a.m., took place against a backdrop of a giant image of fellow Argentine Ernesto "Che" Guevara, which looms nearby on the nation's Ministry of the Interior building to the right of the papal altar.

Towering over the plaza is an obelisk-like monument to Cuban revolutionary hero and Latin American literary icon Jose Martí. The pope referred to Martí in his remarks Saturday.

Before the close of Mass, Pope Francis said he felt compelled to turn his thoughts to Colombia, referring to ongoing talks between that country's government and leftist guerrilla group FARC. The two parties are working on a transitional justice agreement that could change FARC's status from an armed organization to a legitimate political movement.

"May the blood shed by thousands of innocent people during long decades of armed conflict, united to that of the Lord Jesus Christ crucified, sustain all the efforts being made . . . to achieve definitive reconciliation," the pope said in prepared remarks translated by the Vatican. He continued: "May the long night of pain and violence . . . with the support of all Colombians, become an unending day of concord, justice, fraternity and love . . . so that there may be lasting peace. Please, we do not have the right to allow ourselves yet another failure on this path of peace and reconciliation."

Pope Benedict XVI also held a Mass in the plaza during his visit to the island nation in 2012.

Sunday afternoon, Francis was to meet with Raúl Castro before celebrating vespers -- an evening prayer service -- at the Cathedral of Havana.

Francis was scheduled to conclude his public appearances with a greeting Sunday evening to young people at the Centro Cultural Padre Félix Varela.

With Jennifer Barrios and The Associated Press