Pope Francis opens his first full day in Cuba on Sunday with what normally would be the culminating highlight of a papal visit: Mass before hundreds of thousands of people in Havana's evocative Revolution Plaza.

The morning Mass kicks off a busy day for Francis, including a formal meeting with President Raul Castro and a likely encounter with his 89-year-old brother, Fidel. Francis will finish the day with an evening vespers service in the San Cristobal cathedral and meet with Cuban young people.

Young and old turned out in droves Saturday to line Francis' motorcade route and welcome a man that many credit with helping bring about the thaw in relations between Cuba and the United States.

"This visit is like a breath of hope blowing over Cuba," Diego Carrera, a retiree, said as he awaited Francis at the start of a 10-day trip to the onetime Cold War adversaries.

Francis wrote a personal appeal to Presidents Barack Obama and Raul Castro and hosted their delegations at a secret meeting at the Vatican last year to seal a deal after 18 months of closed-door negotiations. Since then, the two leaders have reopened embassies in each other's countries, held a personal meeting and at least two phone calls and launched a process aimed at normalizing ties in fields ranging from trade to tourism to telecommunications.

Upon his arrival, Francis plunged head-first into the rapprochement, urging the Cuban and U.S. governments to push forward on their newly forged path and "develop all its possibilities."

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.

Standing with Raul Castro by his side, Francis said the developments over recent months have given him hope.

"I urge political leaders to persevere on this path and to develop all its possibilities as a proof of the high service which they are called to carry out on behalf of the peace and well-being of their peoples, of all America, and as an example of reconciliation for the entire world," he said.

Castro, for his part, criticized the embargo as "cruel, immoral and illegal" and called for it to end. But he also thanked Francis again for his role in fostering "the first step" in a process of normalizing relations.

The pope's message on Sunday is likely to be less political and more pastoral.

Francis has said he is coming to Cuba as a messenger of mercy, aiming to give solidarity to a long-suffering people and church.

The island's communist government never outlawed religion per se. But it came close, closing religious schools after Fidel Castro came to power in 1959, expelling priests and sending others to prison or work camps, including the current archbishop of Havana, Cardinal Jaime Ortega.

Castro began easing prohibitions on faith in the 1990s, removing constitutionally enshrined atheism ahead of a visit by Pope John Paul II and reinstating Christmas as a public holiday soon after.

The Catholic Church today has quietly established itself as practically the only independent institution with any widespread influence on the island. Expanding into areas once utterly dominated by the state, the church is providing tens of thousands of people with food, education, business training and even libraries stocked with foreign best-sellers.

But it still is seeking more freedom to spread the faith: Church authorities have long wanted to run full-time private schools and get religious programming on state-run airwaves, both of which the government has resisted

In his airport arrival speech, Francis said he hoped his visit would renew the bonds of friendship so the church can continue to do its job "with the freedom and the means necessary."

While most Cubans are nominally Catholic, fewer than 10 percent practice their faith.

Magaly Delgado, an accountant, said she would be at the Mass on Sunday because "I'm a believer and this pope interests me a lot because of all the change that he's making."

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Associated Press writers E. Eduardo Castillo and Anne-Marie Garcia in Havana and Christine Armario and Andrea Rodriguez in Holguin, Cuba, contributed to this report.

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Nicole Winfield on Twitter: https://twitter.com/nwinfield

Michael Weissenstein on Twitter: https://twitter.com/mweissenstein