Pope Francis headed to Cuba Saturday on the first leg of his most high-profile trip to date, which will also take him to the United States -- highlighting his role in mediating the former Cold War foes' historic thaw.

Landmark speeches to the US Congress and the UN General Assembly await next week on the 78-year-old Argentine's first trip to a global superpower whose policies he has never hesitated to criticise.

But first the leader of the world's 1.3 billion Roman Catholics will become the third pope to touch down in communist Cuba in less than two decades.

Francis, smart in his papal whites and clutching the black leather bag he takes on all his travels, waved a cheerful goodbye before boarding an Alitalia plane and taking off from Rome's Fiumicino airport shortly after 0830 GMT.

He is expected to land at around 2000 GMT in Havana, where hoards of people were already gathering Saturday morning to greet him.

"I never thought I would ever come to Cuba," said Cathy Donn, a 56-year-old American who flew in with a group of 187 Catholics organized by the archdiocese of Miami.

"But I decided to take advantage of the opportunity to come see the pope and see him in a place that I would never be able to see," she told AFP on the eve of Francis's arrival.

The visit has added significance as it comes in the aftermath of the historic US-Cuban rapprochement, which allowed diplomatic relations to be restored thanks to secret contacts that Francis, the first Latin American pope, helped facilitate.

Cuban leader Raul Castro is expected at Jose Marti airport to greet the pope, and Francis is also likely to meet Raul's predecessor and brother Fidel during his three-night stop on the Caribbean island.

Although he rejects as absurd some critics' depictions of him as a veiled Marxist, the pope does share with the Castro brothers a radical critique of global capitalism.

But it does not prevent him from working to coax their one-party regime into greater respect for civil rights and religious freedom.

In a message broadcast on the island ahead of his trip he praised Cubans' spirit in the face of adversity.

"It does me a lot of good and helps me to think of your faith in the Lord, of the spirit with which you confront the difficulties of each day," he said.

- New overture from Washington -

Francis begins his visit with a speech at the airport on arrival, after which he has no public engagements ahead of what is a densely packed schedule for the rest of his stay.

As well as Havana, he is due to visit Holguin and Santiago, birthplace of Castro's revolution, following in the footsteps of his predecessors Benedict XVI and John Paul II, who visited in 2012 and 1998, respectively.

That such a small state has been smothered with papal attention reflects the importance the Vatican attaches to the fate of Cuba's Catholics, who have won greater freedoms as part of a diplomatic process in which the Holy See has also championed the case for an easing of the US embargo on trade and investment ties.

In the run-up to Francis's arrival, the Cuban government announced the release of more than 3,500 prisoners, significantly more than were liberated before previous papal visits.

The United States for its part announced a further loosening of restrictions on business and travel with Cuba.

While he is sure of a warm welcome in Cuba, the outcome of the US leg of Francis's 10th overseas trip looks more uncertain.

For some observers, the dominant themes of Francis's papacy -- concern for the poor, his strong stance in favour of action on global warming and his critique of consumerism -- can be read as an indictment of the American way of life.

That was underlined Friday when Republican Congressman Paul Gosar, a Catholic, declared he would boycott the pontiff's historic address to Congress to protest his "leftist" views.

- Special guest -

And while Francis appears hugely popular with American Catholics, he also arrives amid lingering bitterness over the American Church hierarchy's role in covering up widespread sexual abuse of children by priests.

But he is sure to be treated as a special guest by most Americans, from the prisoners he will meet in Philadelphia to President Barack Obama, who will greet him personally on his arrival Tuesday at Andrews Air Force Base outside Washington.

According to Vatican sources, the most important speeches of his trip will see the pope return to familiar themes: the tyranny of global finance and the evils of the throwaway culture, as well as more consensual questions such as human trafficking, unemployment, war and the persecution of religious and ethnic minorities.

While in New York, the pontiff will preside over a multi-faith service at Ground Zero against terrorism and in memory of the victims of the September 2001 attacks on the United States.

Some 1.5 million people are expected for the final mass of the trip, in Philadelphia at an international festival of Catholic families.

Bells to toll for pope's US arrival
Washington (AFP) Sept 19, 2015 - Pope Francis is set to arrive in Cuba on Saturday for a trip that will take him next week to the United States for the first time.

The following are briefs about the pope's upcoming visit to Washington, New York and Philadelphia:

- Bells, bells, bells -

Fifty-six bells will toll when Pope Francis touches down at Andrews Air Force base outside the US capital Tuesday to begin his first-ever US visit.

The bells belong to the Basilica of the National Shrine of the Immaculate Conception, the largest Roman Catholic church in North America that also ranks among the 10 biggest in the world.

It's here that the pontiff will lead his first mass -- to canonize Franciscan friar and missionary Junipero Serra -- that is expected to draw some 40,000 people.

The basilica, Byzantine-Romanesque in style and topped by a golden and blue dome, is dedicated to the patroness of the United States, the Virgin Mary.

Francis will celebrate the mass on the basilica's east portico from a specially crafted altar that pays homage to the Vatican's recent encyclical letter to bishops on the environment and global warming, with its use of recycled fiberboard and American-sourced poplar and cherry veneer.

- 'Wall of lament' on US-Mexico border -

In California, San Diego Bishop Robert McElroy and other faith leaders will be launching "40 Days of Faithful Action" on Sunday with an interfaith event at the US-Mexico border.

To spotlight the plight of separated immigrant families, they will unveil a so-called Wall of Lament between San Diego and Tijuana, Mexico.

Organizers say they are acting on the pope's "message of compassion, respect for human dignity and love all of God's creation."

They have also planned events in support of Francis's trip to a Philadelphia jail.

- Pontiff's Twitter plea -

"I ask you to join me in praying for my trip to Cuba and the United States. I need your prayers," Pope Francis tweeted Friday.

Twitter is the pontiff's preferred social media platform. Boasting nearly 23 million followers, he uses it to reach out to his flock in nine languages.

- Work from home -

US federal government workers have been encouraged to work from home if possible during the pope's visit to Washington.

The Office of Personnel Management has said that while the government will stay open, agencies are "strongly" encouraged to "allow employees to telework to keep the government operating while helping to minimize traffic congestion and support law enforcement efforts during this event."

If possible, employees are encouraged to use vacation time during the September 22-24 visit.