WASHINGTON -- Pope Francis, a steadfast champion of the poor, arrived Tuesday in the capital of the world's wealthiest nation intent on preaching compassion -- and wading into thorny issues such as immigration and global warming.

In an unusual sign of respect, President Barack Obama personally welcomed the pontiff to America, giving Francis a hearty handshake moments after the papal plane landed.

Francis stepped onto a red carpet at Joint Base Andrews as an honor guard stood in salute and hundreds of admirers in metal bleachers raucously chanted "Welcome to the USA! The Pope, hey-hey!"

It was the first visit to America for Francis and only the fourth ever by a pope, kicking off a six-day, three-city tour that includes addresses to a joint meeting of Congress, the United Nations and the World Meeting of Families in Philadelphia.

In a Vatican news conference Tuesday night, spokesman Federico Lombardi said the pope will touch on immigration and the environment in his United Nations speech on Friday.

Those are "obvious" subjects, Lombardi said, along with the Syrian refugee crisis and climate change. But there were no hints of any surprising topics from the sometimes blunt pontiff, who in a June manifesto called for a revolution to change a "perverse" economic system that exploits the poor and pollutes the planet.

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The pope's arrival followed three days in Cuba packed with speeches, homilies and meetings, so the 78-year-old Francis was not scheduled to make any remarks after his 3:49 p.m. arrival.

After warmly greeting first lady Michelle Obama and daughters Malia and Sasha, Francis met Vice President Joe Biden and his wife Jill, followed by a number of Catholic leaders.

Among the Catholics in attendance at the base, Dorothy Newman, 74, of Largo, Maryland, said seeing Francis on American soil fills her with hope.

"This was a great day," she said. "I just hope the pope continues to pray for us, and especially for all our leaders. ... He's trying to bring people together in love."

Waving to supporters, Francis climbed into the backseat of a modest black Fiat with license plate SCV 1 for Vatican City. A motorcade took him to the Apostolic Nunciature on Massachusetts Avenue, the equivalent of an embassy, where he is staying during his Washington visit.

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At the Nunciature, children waved Vatican flags -- gold and white, with the keys of St. Peter and the papal tiara. Young people sang, and played drums and tambourines.

Eddie Velasquez, a Catholic from Rockville, Maryland, said he came to show "how thankful we are for our leaders of the church, especially the Holy Father."

During the flight to Washington, Francis told reporters that any notion he is left-leaning is a misunderstanding and that he adheres to church doctrine.

"Maybe there's an impression I'm a little bit more leftie, but I haven't said a single thing that's not in the social doctrine of the church," Francis said during a 30-minute news conference, alternating between Spanish and Italian.

The Argentine-born Francis, the first Latin American pope, said he had no knowledge that dissidents were detained during his visit to Cuba, as some news reports indicated, and denied refusing to meet with them.

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"I don't have any news that that has happened," he said of the reported arrests. "There was no meeting planned with the dissidents."

He said he had to turn down a number of requests for private audiences, including one from a "head of state" -- presumably a reference to Argentine president Cristina Kirchner, who was in Cuba for the pope's visit.

The pope said he does not plan to talk specifically about the U.S. embargo of Cuba during his address to Congress, though he may mention the importance of bilateral agreements.

Earlier in the day, during a homily at Mass in El Cobre, the most sacred Catholic shrine in Cuba, Francis delivered more of his message that the church must play an active role in solving the world's problems.

"Generation after generation, day after day, we are asked to renew our faith," he said at the Minor Basilica of the Virgin of Charity. "We are asked to live the revolution of tenderness as Mary, our Mother of Charity, did. We are invited to 'leave home' and to open our eyes and hearts to others.

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" . . . We want to be a Church which goes forth to build bridges, to break down walls, to sow seeds of reconciliation. Like Mary, we want to be a Church which can accompany all those 'pregnant' situations of our people, committed to life, to culture, to society, not washing our hands but rather walking with our brothers and sisters."

In Washington, the pope is to lead a midday prayer Wednesday at the Cathedral of St. Matthew the Apostle and an afternoon Mass for the canonization of evangelizer Junipero Serra at the Basilica of the National Shrine of the Immaculate Conception. His address before Congress is tomorrow morning.

Anticipation has been building for Francis' visit here.

Along Connecticut Avenue NW, several miles from the Capitol and the White House, nearly every streetlight pole bore the grinning face of Francis, an invitation to tomorrow's rally at the National Mall in support of his call for "moral action on climate."

A storefront displayed merchandise, including T-shirts with bright colors and messages such as "I k Pope Francis."

One restaurant's sidewalk menu sign simply declared: "The POPE is coming."