Washington (CNN) Pope Francis immediately dove into the whirlpool of U.S. politics on Wednesday, using his first direct address to the nation to weigh in on deeply divisive issues including climate change, Cuba, and traditional marriage as he was greeted by exuberant crowds packing the streets of Washington.

The pontiff lived up to his reputation for blunt talk at a welcoming ceremony at the White House, introducing himself as the son of the kind of "immigrant family" on which America was built -- a clear reference to the controversy swirling around millions of undocumented people in the country.

Speaking in English, the Argentine-born Francis also said he was ready to listen to the "hopes and dreams of the American people" and to offer guidance to those charged with shaping the nation's political future "in fidelity to its founding principles."

And in comments that could antagonize Republicans, Francis endorsed President Barack Obama's efforts on climate change and rebuilding ties with Cuba after more than half a century of estrangement.

Wednesday, which began with pomp and politics and ended with a controversial canonization, was the Pope's first full day in the United States. The 6-day visit will take him later this week to New York, where he will address the United Nations, and Philadelphia for the World Meeting of Families, a large Catholic event that is expected to draw nearly a million pilgrims to papal Masses.

Photos: Pope Francis in the United States Pope Francis waves from the top of the steps as he prepares to depart Philadelphia International Airport on Sunday, September 27, on his way back to Rome. The Pope has been on a six-day visit to the United States, with stops in Washington, New York and Philadelphia. Hide Caption 1 of 50 Photos: Pope Francis in the United States Pope Francis greets families at the conclusion of Mass at the World Meeting of Families at Benjamin Franklin Parkway in Philadelphia on September 27. Hide Caption 2 of 50 Photos: Pope Francis in the United States A huge crowd gathers to celebrate Mass with Pope Francis on September 27 in Philadelphia. Hide Caption 3 of 50 Photos: Pope Francis in the United States Pope Francis celebrates Mass at the World Meeting of Families at Benjamin Franklin Parkway in Philadelphia on September 27. Hide Caption 4 of 50 Photos: Pope Francis in the United States Pope Francis waves to the crowd from the popemobile during a parade along the Benjamin Franklin Parkway before Mass on September 27 in Philadelphia. Hide Caption 5 of 50 Photos: Pope Francis in the United States Pope Francis waves to the audience as he leaves Saint Charles Borromeo Seminary in Wynnewood, Pennsylvania, after addressing bishops on September 27. Hide Caption 6 of 50 Photos: Pope Francis in the United States Aretha Franklin performs as Pope Francis looks on during the Festival of Families on September 26 in Philadelphia. Hide Caption 7 of 50 Photos: Pope Francis in the United States Pope Francis takes the stage at the Festival of Families on September 26 in Philadelphia. Hide Caption 8 of 50 Photos: Pope Francis in the United States Pope Francis speaks in front of Independence Hall, from the lectern used by President Abraham Lincoln during the Gettysburg Address, on Saturday, September 26, in Philadelphia. Hide Caption 9 of 50 Photos: Pope Francis in the United States Pope Francis rides in the Popemobile along Independence Mall before delivering a speech outside Independence Hall on September 26. Hide Caption 10 of 50 Photos: Pope Francis in the United States Residents in decorated apartment windows await the arrival of Pope Francis near Independence Mall on September 26. Hide Caption 11 of 50 Photos: Pope Francis in the United States Pope Francis blesses Communion during a Mass at Cathedral Basilica of Saints Peter and Paul, Saturday, September 26, in Philadelphia. Hide Caption 12 of 50 Photos: Pope Francis in the United States Pope Francis delivers Mass before a crowd of more than 2,000, consisting largely of priests, women religious and deacons on September 26. Hide Caption 13 of 50 Photos: Pope Francis in the United States Members of the choir arrive at Cathedral Basilica of Saints Peter and Paul prior to the arrival of Pope Francis on September 26. Hide Caption 14 of 50 Photos: Pope Francis in the United States Pope Francis kisses and blesses Michael Keating, 10, of Elverson, Pennsylvania, after arriving in Philadelphia on September 26. Keating has cerebral palsy and is the son of Chuck Keating, director of the Bishop Shanahan High School band, which performed at Pope Francis' airport arrival. Hide Caption 15 of 50 Photos: Pope Francis in the United States The faithful receive communion at the snack bar during Mass at Madison Square Garden on September 25. Hide Caption 16 of 50 Photos: Pope Francis in the United States Pope Francis celebrates Mass at Madison Square Garden on Friday, September 25, in New York City. Hide Caption 17 of 50 Photos: Pope Francis in the United States A crucifix hangs above members of the clergy during Mass at Madison Square Garden on September 25. Hide Caption 18 of 50 Photos: Pope Francis in the United States Pope Francis rides through New York's Central Park on September 25. Francis addressed the U.N. General Assembly and will head to Philadelphia this weekend for the World Meeting of Families, a large Catholic event expected to draw nearly 1 million pilgrims. Hide Caption 19 of 50 Photos: Pope Francis in the United States Pope Francis greets the crowd in New York's East Harlem Neighborhood on September 25. Hide Caption 20 of 50 Photos: Pope Francis in the United States Pope Francis places a white rose at the 9/11 memorial in New York on September 25. Hide Caption 21 of 50 Photos: Pope Francis in the United States Francis attends a multireligious service at the 9/11 memorial. Hide Caption 22 of 50 Photos: Pope Francis in the United States The Pope arrives at the U.N. General Assembly to give his speech. Hide Caption 23 of 50 Photos: Pope Francis in the United States Francis gives a thumbs-up after leading an evening prayer service Thursday, September 24, at St. Patrick's Cathedral in New York. Hide Caption 24 of 50 Photos: Pope Francis in the United States Nuns celebrate at St. Patrick's Cathedral as they wait for the arrival of Pope Francis. Hide Caption 25 of 50 Photos: Pope Francis in the United States Francis greets crowds as he arrives at St. Patrick's Cathedral. Hide Caption 26 of 50 Photos: Pope Francis in the United States Crowds welcome Pope Francis to New York on September 24 after his arrival at John F. Kennedy International Airport. Hide Caption 27 of 50 Photos: Pope Francis in the United States A Pope Francis doll amuses the man himself at John F. Kennedy International Airport. Hide Caption 28 of 50 Photos: Pope Francis in the United States Pope Francis stops to look at a sculpture of Spanish-born missionary Junipero Serra at the U.S. Capitol in Washington on September 24. The Pope canonized Serra during a Mass the day before. Hide Caption 29 of 50 Photos: Pope Francis in the United States Members of Congress accompany Pope Francis on the Speaker's Balcony on Capitol Hill on September 24. Hide Caption 30 of 50 Photos: Pope Francis in the United States Congressional staffers and guests strain to view and photograph the Pope at the Capitol. Hide Caption 31 of 50 Photos: Pope Francis in the United States Francis blesses a child at the Capitol. Hide Caption 32 of 50 Photos: Pope Francis in the United States Pope Francis addresses a joint meeting of the U.S. Congress on September 24. Vice President Joe Biden and House Speaker John Boehner sit behind him in the House chamber. Hide Caption 33 of 50 Photos: Pope Francis in the United States Francis is the first Pope to address a joint meeting of Congress. Hide Caption 34 of 50 Photos: Pope Francis in the United States Boehner welcomes Pope Francis before his speech to Congress. Hide Caption 35 of 50 Photos: Pope Francis in the United States Pope Francis canonizes Serra during a Mass in Washington on Wednesday, September 23. Hide Caption 36 of 50 Photos: Pope Francis in the United States The Pope celebrates Mass at the National Shrine of the Immaculate Conception on September 23. Hide Caption 37 of 50 Photos: Pope Francis in the United States Nuns wait for Pope Francis to arrive inside the Basilica of the National Shrine of the Immaculate Conception. Hide Caption 38 of 50 Photos: Pope Francis in the United States Pope Francis greets crowds during his parade in Washington on September 23. Hide Caption 39 of 50 Photos: Pope Francis in the United States Pope Francis reaches out to bless a child during the parade in Washington. Hide Caption 40 of 50 Photos: Pope Francis in the United States President Barack Obama hosts the Pope in the Oval Office of the White House on September 23. Hide Caption 41 of 50 Photos: Pope Francis in the United States Obama, the Pope and first lady Michelle Obama greet the crowd during an arrival ceremony on the South Lawn of the White House on September 23. Hide Caption 42 of 50 Photos: Pope Francis in the United States Pope Francis addresses guests on the South Lawn of the White House. Hide Caption 43 of 50 Photos: Pope Francis in the United States People wave U.S. and Vatican flags as they wait for the Pope's arrival at the White House on September 23. Hide Caption 44 of 50 Photos: Pope Francis in the United States Early on September 23, people gather along Pope Francis' parade route in Washington. Hide Caption 45 of 50 Photos: Pope Francis in the United States Pope Francis departs from Andrews Air Force Base in Maryland shortly after his flight landed on Tuesday, September 22. Hide Caption 46 of 50 Photos: Pope Francis in the United States Pope Francis waves from his car at Andrews Air Force Base. Hide Caption 47 of 50 Photos: Pope Francis in the United States Pope Francis is escorted by the Obamas and their daughters after arriving in the country. Hide Caption 48 of 50 Photos: Pope Francis in the United States Clergy members brace for the wind as they stand on the tarmac at Andrews Air Force Base. Hide Caption 49 of 50 Photos: Pope Francis in the United States The Pope waves at Obama and his family. Hide Caption 50 of 50

He told the president that it was "encouraging that you are proposing an initiative for reducing air pollution. Accepting the urgency, it seems clear to me also that climate change is a problem that can no longer be left to a future generation

"When it comes to the care of our 'common home' we are living at a critical moment of history," he said.

Francis is using the global platform offered by his first-ever visit to the United States to emphasize the theme at the center of his two-year-old papacy: challenges like climate change, income inequality and the plight of immigrants are moral -- not political -- issues. And, he said, the richest, most developed countries have an obligation to act.

"I would like all men and women of good will in this great nation to support the efforts of the international community to protect the vulnerable in our world and to stimulate integral and inclusive models of development," Francis said, in comments that reflect what he sees as a more inclusive vision of capitalism.

While some of these themes were sure to please the left, he also delivered a firm defense of traditional values, warning that the institution of marriage and family needed to be protected at "a critical moment in the history of our civilization." Those remarks could irk liberals months after the Supreme Court ruled in favor of same-sex marriage nationwide.

He said that it was right that society was "tolerant and inclusive" but warned that American Catholics were "concerned that efforts to build a just and wisely ordered society respect their deepest concerns and their right to religious liberty. That freedom remains one of America's most precious possessions."

Obama gently -- but pointedly -- argued that "here in the United States, we cherish religious liberty."

Obama and first lady Michelle Obama earlier greeted the Pope at the White House as he stepped out of his small black Fiat, which he is using to make a statement of humility in Washington, a city full of limos and hulking government SUVs. In another nod to Francis' pared down style, the White House dispensed with the 21-gun salute to which he is entitled as the titular head of the Vatican state.

Photos: Photos: Few images exist of 18th-century Spanish missionary Junipero Serra, a founding figure of the American West. This portrait has become one of the standard representations of him and was done in the early 1900s by a Mexican priest, Father Jose Mosqueda, who said he copied it from a work that could have been an original portrait of Serra from the 1750s. Hide Caption 1 of 5 Photos: This is Serra's personal "novena" prayer book, which is kept in the archives of Old Mission Santa Barbara. The first page is in Spanish. Hide Caption 2 of 5 Photos: Here's another image of Junipero Serra from a first-edition book about the Spanish Franciscan friar entitled, "Relacion Historica de la Vida y Apostolica Tareas del Venerable Padre Fray Junipero Serra," by Father Francisco Palou. His book was first published in Mexico in 1787. Hide Caption 3 of 5 Photos: This is how Father Serra signed his name on a letter about the value of prayer to another Franciscan friar, Fermin Francisco de Lasuen. Hide Caption 4 of 5 Photos: Serra often corresponded about the California missions as he evangelized Native Americans in the late 1700s. This letter is written by Serra's own hand and is kept in a climated-controlled vault at Old Mission Santa Barbara. Hide Caption 5 of 5

On a glorious early-fall morning, the President and Pope stood together before an honor guard as a band played the national anthems for the Vatican and the U.S.

Obama paid warm tribute to the Pope as an individual as well as the leader of 70 million U.S. Catholics, saying he displayed "unique qualities" of a leader "whose moral authority comes not just through words but also through deeds."

The president warmly welcomed the pontiff's support on climate change and Cuba, despite the White House saying Obama wouldn't use the visit to build domestic political support for these issues.

"Holy Father, we are grateful for your invaluable support of our new beginning with the Cuban people, which holds out the promise of better relations between our countries, greater cooperation across our hemisphere, and a better life for the Cuban people," Obama said.

The president, who met the Pope for one-on-one talks in the Oval Office, presented his guest with a sculpture of an ascending dove made from metal taken from the Statue of Liberty and wood which once grew in the White House garden.

Francis also took time to minister to the hurting U.S. Catholic Church, meeting bishops at The Cathedral of St. Matthew the Apostle in Washington and offering counsel at a time of great upheaval among his American flock which has thinned partly because of child sexual abuse scandals.

"I know how much the wounds of these last few years have weighed on your spirit, and I have supported your generous commitment to bring healing to victims -- in the knowledge that in healing, we too are healed -- and to work to ensure that such crimes will never be repeated," Francis said.

The Pope's visit brought Washington to a halt. Massive crowds greeted the pontiff as he emerged from the White House complex, standing five or six deep on sidewalks. Several times, his vehicle, a converted white Jeep Wrangler, slowed so the Pope could bless a small child handed to him by a security agent.

In front of and behind the Popemobile, police motor cyclists and black armored Secret Service vehicles fanned out -- evidence of a massive security operation being mounted during the Pope's visit.

He took to the streets again as he headed to a mass to canonize 18th Century Hispanic missionary Junipero Serra who is said to have taken Christianity to California but who is controversial because he is reputed to have treated Native Americans poorly.

Some people had been waiting to see the Pope for hours. At St Matthew's, for instance, 58-year-old Dolores Reyes was beaming as she wore a T-shirt that read: "Caminando con el Papá Francisco" -- Walking with Pope Francis.

"He seems different," she said. "He is more humble, more connected with people, with everyone. He is a great Pope. I love his approach."

Earlier, outside the White House, Adriana Cazorla, from Washington state, listened intently as Pope Francis spoke on the big screen and applauded when he said he was a son of immigrants.

"We want the pope to know that 11 million undocumented people are being treated like criminals in this country," Cazorla said before Francis made his remarks.

Francis' next political intervention could come on Thursday when he makes an address to a joint meeting of Congress. He will travel to New York and Philadelphia later in the week to wrap up his six-day visit.