Pope Francis, speaking on the White House lawn Wednesday morning alongside President Obama and in front of thousands, delivered his first message to the United States, saying he was looking forward "to these days of encounter and dialogue."

"I am deeply grateful for your welcome in the name of all Americans. As the son of an immigrant family, I am happy to be a guest in this country, which was largely built by such families. I look forward to these days of encounter and dialogue, in which I hope to listen to, and share, many of the hopes and dreams of the American people," Francis said.

In his address, the Argentinian touched on the topics of climate change and religious freedom, immediately putting two controversial topics at the forefront of his visit.

"With countless other people of goodwill, [American Catholics] are likewise concerned that efforts to build a just and wisely ordered society respect their deepest concerns and their right to religious liberty. That freedom reminds one of America's most precious possessions ... all are called to be vigilant, precisely as good citizens, to preserve and defend that freedom from everything that would threaten or compromise it," Francis said, turning to look at Obama as he spoke.

The pontiff also praised Obama for his work in combating climate change.

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"Mr. President, I am finding it 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 which can no longer be left to future generations. When it comes to the care of our common home we are living at a critical moment of history," Francis said to applause from the gathered crowd.

Officials said that 11,000 ticketed guests were present, not including press, staff, military or volunteers.

Before leaving for the White House, Pope Francis took his time greeting schoolchildren outside the Vatican's diplomatic mission in Washington where he spent the night. 

Obama formally greeted the pontiff with first lady Michelle, saying “it is my great honor and privilege to welcome you to the United States of America.”

“You remind us that in the eyes of God our measure as individuals, and as societies, is not determined by wealth or power or station or celebrity, but by how well we hew to Scripture’s call to lift up the poor and the marginalized, to stand up for justice and against inequality, and to ensure that every human being is able to live in dignity – because we are all made in the image of God,” Obama said.

"You shake our conscience from slumber; you call on us to rejoice in Good News, and give us confidence that we can come together, in humility and service, and pursue a world that is more loving, more just, and more free.  Here at home and around the world, may our generation heed your call to 'never remain on the sidelines of this march of living hope,'" he said.

The welcoming ceremony was just the start of a full day of events for Francis, including a highly anticipated address to U.S. bishops at the Cathedral of St. Matthew the Apostle and the first ever canonization Mass held on American soil. Shortly after the ceremony, Francis and Obama had a sit-down meeting and Obama gave the pontiff a tour of the White House.

Francis then delighted an adoring crowd with his popemobile procession in Washington. The pope moved slowly past throngs lining his route from the White House. At one point, a young girl carrying a yellow banner got outside the police barricade holding the crowds back and tried to approach the popemobile. She shied back when a bodyguard came near to pick her up and bring her to Francis. But then the pope gestured to her to come to him, and she allowed the bodyguard to pick her up and bring her to Francis for a papal kiss and blessing.

Francis arrived in Washington, the first stop on his six-day, three-city tour of the U.S., late Tuesday afternoon. He was greeted at Andrews Air Force Base by Obama, Vice President Joe Biden, their wives, and Obama's daughters before being driven to the diplomatic mission where he will spend his time in the nation's capital. 

Even before he arrived in America, Francis was fending off conservative criticism of his economic views. He told reporters on his flight from Cuba that some people may have an inaccurate impression that he is "a little bit more left-leaning."

"I am certain that I have never said anything beyond what is in the social doctrine of the church," he said.

As for conservatives who question whether he is truly Catholic, he added jokingly, "If I have to recite the Creed, I'm ready."

Obama is anxious to add Francis' undeniable star power to his own efforts on climate change and income inequality, among other things, by finding common cause with the pope. The two differ sharply on other issues, such as abortion and same-sex marriage.

From Francis' vantage point, his next stop after the White House is perhaps more critical. The 78-year-old pontiff will meet with America's 450-strong bishops' conference at midday.

The first pope from the Americas also was acting Wednesday to canonize a Spanish friar who brought the Catholic faith to California.

Francis was to celebrate the Mass of canonization for Junipero Serra in Spanish. Several thousand of the 25,000 tickets to the event were set aside for Spanish-speaking people, many from California. The Basilica of the National Shrine of the Immaculate Conception erected a temporary sanctuary on the east portico for the Mass.

On Thursday, Francis planned to deliver the first papal address ever to Congress, speaking to Republican-majority legislators deeply at odds with Obama on issues such as gay rights, immigration, abortion and climate change. Those same issues are roiling the early months of the presidential campaign.

For all the focus on Francis' speeches, his less scripted moments in meeting with immigrants, prisoners and the homeless could prove more memorable.

He was expected to meet with poor immigrants and other clients of Catholic Charities in Washington and with prisoners in Pennsylvania. He also is known to veer off schedule for unscripted encounters.

The Associated Press contributed to this report.