Washington - An estimated 15 000 people crowded onto the White House's South Lawn on Wednesday to hear Pope Francis and President Barack Obama speak to the urgency of tackling climate change in one of the most unusual White House ceremonies in recent times.
Francis was received with full military honours and treated to a performance by the Army's classic Fife and Drum Corps at the special ceremony.
"Our backyard is not typically this crowded," Obama said.
In their remarks on the South Lawn, both leaders dug into the climate change issue, with Francis applauding Obama's efforts to reduce "air pollution" - a reference to greenhouse gasses blamed for global warming.
"We are living at a critical moment of history," Francis said. "Climate change is a problem which can no longer be left to a future generation."
Francis made his remarks ahead of an appearance Thursday before Congress, where conservative Republicans - many of them climate change sceptics - control both the House of Representatives and the Senate.
Francis said he was hopeful that there was still time to change course on the issue - an apparent reference to December climate talks in Paris that are to set the world on a new path of reducing greenhouse gasses.
Obama said he supported Francis' call "to come together to preserve our precious world for future generations."
Pope Francis also touched on the issue of immigration, saying he himself was an immigrant and that the US was "largely built by such families."
Immigration is another contentious topic for opposition Republicans, who have resisted Obama's efforts to reform the system.
Francis praised Obama's efforts "to mend broken relationships and to open new doors" - an apparent referrence to the rapprochement with Cuba that Francis himself helped broker.
Obama made frequent references to his own Christian beliefs, using the public platform to reject the most recent round of charges by Republican critics that he is Muslim.
The large enthusiastic crowd waved US and Vatican flags, with several Argentinian flags being waved in tribute to the pontiff's heritage.
"It's a special event," said Aiden Nurse, age 9. "It's my first pope."
His mother, Shamena Nurse, 34, told dpa that Francis was "definitely appealing to this generation and this time, and to be here with my son is very special."
After leaving the White House and his one-on-one meeting with Obama, the 78-year-old pontiff rode his popemobile in a parade around the Ellipse, a park adjoining the White House, so that those without tickets to the special events in Washington could catch a glimpse of the Catholic leader.
About 200,000 visitors were expected, and he stopped along the way in the open vehicle to great screaming fans.
Black Fiat 500L
In the afternoon, Francis is due to attend a prayer service with 300 US bishops before leading the canonization mass for Franciscan friar Junipero Serra (1713-84), who founded nine missions in California.
Twenty-five thousand people are expected at the canonization ceremony, to take place at the Basilica of the National Shrine of the Immaculte Conception.
After a four-day visit to Cuba, Francis arrived Tuesday at a military airport outside the US capital to begin his six-day tour of Washington, New York and Philadelphia.
In another departure from tradition on the pope's visit, Obama met him at the airport. Francis then climbed into the back seat of a black Fiat 500L with an Italian license plate - an unusually humble car for a Washington motorcade - before his 30-minute, high-security drive to the Vatican embassy, the nunciature, to spend the night.
The White House has underlined Obama's and Francis' agreement on social issues - such as combating climate change and inequality - but Obama's policies on abortion and same-sex marriage clash with Catholic doctrine.
Francis is the Vatican head of state, but the White House has stressed that their meeting is not a political event but rather a chance to emphasize shared values.