SANTIAGO, CUBA -- Pope Francis spoke of the power of transformation during the last full day of his travels through Cuba Monday, while also acknowledging "the efforts and the sacrifices" the Catholic Church has faced in this Communist-run nation.

Francis began Monday in the eastern city of Holguín, where he celebrated a morning Mass in that city's Plaza of the Revolution. Tens of thousands of Cubans there braved punishing heat so scorching that it sent dozens to relief tents set up by the Red Cross.

Many who attended -- some arriving hours in advance -- said they were enthralled with the pontiff.

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"He's doing something very good, he's fighting for unity, for peace, for tranquillity," Marina Rabi, 75, said in Spanish.

In his homily, Francis drew on the New Testament story of Matthew, in which the tax collector heeded Jesus' call to abandon his post and become a servant of others.

Jesus "invites us slowly to overcome our preconceptions and our reluctance to think that others, much less ourselves, can change," Francis said. "He challenges us daily with the question: 'Do you believe? Do you believe it is possible that a tax collector can become a servant? Do you believe it is possible that a traitor can become a friend? Do you believe it is possible that the son of a carpenter can become the Son of God?' "

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The pope also spoke of the travails the Catholic Church has faced in Cuba, especially in far-flung areas.

"I know the efforts and the sacrifices being made by the Church in Cuba to bring Christ's word and presence to all, even in the most remote areas," he said.

Francis went on, lauding especially the " 'mission houses' which, given the shortage of churches and priests, provide for many people a place for prayer, for listening to the word of God, for catechesis and community life."

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An estimated 60 percent of Cubans identify as Catholic. And while President Raúl Castro has allowed more church freedom in Cuba and the Cuban constitution officially allows religious freedom, practically speaking, some restrictions remain, according to experts and the U.S. State Department.

Dissidents said to be detained

Several dissidents reported being detained by Cuban officials as they tried to accept the Vatican's invitation to meet the pope at a Mass Saturday in Havana.

"It really shows the intolerance of the totalitarian Castro regime and the lack of respect to the Vatican, which invited us," said Berta Soler, head of the well-known opposition group Ladies in White.

Vatican spokesman Federico Lombardi confirmed Monday that some dissidents were invited to greet the pope, but said he was unaware of the reason why it didn't happen, and Cuban officials have not issued any public statements regarding dissident activity.

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Francis also made another public appearance with a blessing of Holguín from Loma de la Cruz, a hill that features a large wooden cross that overlooks the city.

It was the first papal visit to the city of about 300,000, and local church officials said they were surprised yet elated he made the trip.

"Holguín isn't typically the type of city a pope would visit," Irina Montero, a press official for the Diocese of Holguín, said in Spanish. "For us this has been incredible. From the time it was announced we haven't stopped giving thanks."

Monday afternoon, Francis flew here -- Cuba's second-largest city and home to the country's patron saint, Our Lady of Charity of El Cobre.

He met with bishops at St. Basil the Great Seminary before ending his public activities for the day by praying before an icon at the shrine of the patron saint.

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The pontiff brought a bouquet of flowers that he placed before the foot-tall wooden statue of the virgin and stood in prayer for a few minutes.

He declined to kneel down on a kneeler that had been placed in front of the virgin and a chair was brought out for him instead. Francis, 78, suffers from sciatica and a bad knee, and Lombardi acknowledged Monday that the pope has been walking with difficulty during the trip, particularly on stairs.

His calling 62 years ago

Monday also marked a personal anniversary for Francis: It was 62 years ago on Sept. 21 that 17-year-old Jorge Mario Bergoglio -- as the pope then was known -- decided to become a priest. At confession at his parish church in the Flores neighborhood of Buenos Aires, he later wrote, he "realized God was waiting for me."

Tuesday, Francis is set to spend the morning in Santiago, where he is to celebrate Mass and meet with local families before boarding a special Alitalia flight to Washington, D.C.

He arrives at Andrews Air Force Base at 4 p.m. Eastern Tuesday.

The pope's nine-day visit to Cuba and the United States will culminate with his appearance in Philadelphia at a million-person Mass as part of the World Meeting of Families.

The Vatican Monday released a 22-second video clip of Francis, taped in Vatican City, in which the first Latin American pope beseeched pilgrims to head to Philadelphia for the event.

"I look forward to greeting the pilgrims and the people of Philadelphia when I come for the World Meeting of Families," he said in heavily accented English. "I will be there because you will be there. See you in Philadelphia!"

With AP