WASHINGTON (Reuters) - Pope Francis on Wednesday urged the United States to help tackle climate change and touched on other divisive U.S. political issues such as immigration and economic inequality 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 to the church.

"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.

In an era of renewed racial tensions in the country, the 78-year-old Pope invoked America's best known civil rights leader, the late Rev. Martin Luther King, Jr. to make points about the environment and equality.

Speaking later to American bishops, Francis acknowledged the sexual abuse scandal that has tainted the U.S. Church for years by saying these "crimes" should "never be repeated."

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 pope depart from his usual practice and give a speech in English.

He again made a gesture of simplicity, pulling up 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. The pope also commented on immigration and religious freedom, issues that were on the U.S. political agenda in the run up to the November 2016 presidential election.

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 held talks in the White House, where they also discussed refugees. 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."

ADORING CROWDS

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.

Maria Manzo, a 79-year-old Catholic from New Jersey, said hearing the pope on the South Lawn was "just awesome," but disagreed with his raising of political issues.

"I don't like to see the Church get involved in stuff like that," she said. "I think we've got enough problems of our own."

Adoring crowds waving white-and-yellow Vatican flags cheered the pontiff on a parade through Washington streets.

At one point, he had his converted Jeep stop when he saw a baby in the crowd. Francis was handed the five-month-old son of Salvadoran immigrants, and kissed him on the head.

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.

While Francis has pushed for a more inclusive Church, appealing to divorced Catholics and gays, he has not changed teaching that opposes abortion and same-sex marriage.

A small group of women protested outside St Matthew's Cathedral calling on the pope to allow women priests.

"Pope Francis: Ordain women," read a sign they held.

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.

Later on Wednesday, he will canonize 18th century Spanish missionary Friar Junipero Serra over the objections of critics who say that Serra suppressed Native American cultures in California.

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.

(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)