By Philip Pullella and Jeff Mason
WASHINGTON, Sept 23 (Reuters) - Pope Francis on Wednesday urged the United States to help tackle climate change and called on Americans to build a truly tolerant and inclusive society, as he struck a political tone on his first visit to the world's richest nation.
In a speech on the White House South Lawn, the Argentine pontiff lauded President Barack Obama's efforts to reduce air pollution, months after Francis made the environment one of his top issues by issuing a landmark "encyclical" letter.
"It seems clear to me also that climate change is a problem which 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," the pope said at a welcoming ceremony.
Apart from the environment, Francis touched on several issues on the U.S. political agenda in the run up to the November 2016 presidential election, such as immigration, inequality and religious freedom.
In an era of renewed racial tensions in the country, he invoked America's best known civil rights leader, the late Rev. Martin Luther King, to make points about the environment and equality.
As Washington streets were closed and federal workers stayed home, about 15,000 people gathered in bright sunshine on the South Lawn. They heard the 78-year-old pope depart from his usual practice and give a speech in English.
He again made a gesture of simplicity, pulling up beside the South Lawn in the small black Fiat car he used for his arrival in the U.S. capital on Tuesday.
Francis is a frequent critic of the damage caused to the world's poor and the environment by capitalism's excesses.
Obama, whose plans for a climate change bill were thwarted in Congress early in his presidency, said he shared the pope's concerns about the environment.
"Holy Father, you remind us that we have a sacred obligation to protect our planet - God's magnificent gift to us. We support your call to all world leaders to support the communities most vulnerable to a changing climate and to come together to preserve our precious world for future generations," Obama said.
Francis and Obama headed into the White House for talks. Both men see eye-to-eye on climate change and defense of the poor but hold different views on abortion rights and gay marriage.
Francis gave his support to traditional marriage, pointing out that he will travel to Philadelphia later in his six-day visit to the United States for a meeting of Catholics "to celebrate and support the institutions of marriage and the family."
The leader of the world's 1.2 billion Roman Catholics, Francis sprinkled some stardust in a city that is not unduly impressed with important foreign visitors.
"We love you Pope Francis!" one person shouted as the White House ceremony ended. The crowd of invited guests erupted in cheers.
"No words can describe it. It's just awesome," said Maria Manzo, 79, of New Jersey who attended the event.
Adoring schoolchildren cheered the pontiff when he left the Vatican Embassy on Wednesday morning. Thousands of the faithful gathered ten deep at places to greet him on a parade through the streets of downtown Washington later in the day.
The first Latin American pope, Francis has electrified liberal-leaning U.S. Catholics, Democrats and many non-Catholics with a shift in emphasis toward concern for the poor and immigrants and his appeals to combat climate change. But his criticism of unbridled capitalism has unsettled U.S. conservatives.
In an apparent nod to the fierce debate in the United States about illegal immigration, Francis described himself as the son of an immigrant Italian family in Argentina. "I am happy to be a guest in this country, which was largely built by such families," he said at the White House.
Francis said on Tuesday that his teachings on economic fairness and climate change are "all in the social doctrine of the Church."
In his White House speech, he paraphrased King's 1963 "I Have A Dream" speech, saying that the world has "defaulted on a promissory note" to the planet and millions of marginalized people.
Francis will give the first speech by any pope to the U.S. Congress on Thursday, an address to the U.N. General Assembly in New York on Friday and an open-air Mass in Philadelphia where 1.5 million people are expected to attend.
The pope speaks to U.S. bishops on Wednesday then celebrates Mass at the one of the most important Roman Catholic churches in the United States, the Basilica of the National Shrine of the Immaculate Conception in Washington.
There, he will canonize 18th century Spanish missionary Friar Junipero Serra over the objections of critics who say that Serra suppressed Native American cultures in what is now California.
(Additional reporting by Scott Malone, Julia Edwards, Susan Cornwell, Ian Simpson and Roberta Rampton; Writing by Alistair Bell; Editing by Mary Milliken and Grant McCool)