WASHINGTON – Barack Obama began his second term as president Monday with a stirring call for progressive values and action on climate change and immigration reform, as well as a historic presidential affirmation of gay rights.

In an inaugural address before hundreds of thousands on a cold, cloudy morning, the Democratic president called on the nation to “answer the call of history” and unify behind a vision that offers hope to the humblest and equality for all.

“We are true to our creed when a little girl born into the bleakest poverty knows that she has the same chance to succeed as anybody else, because she is an American; she is free, and she is equal, not just in the eyes of God but also in our own,” Obama said.

It was a lofty speech that contrasted sharply with Obama’s subdued words of four years ago, when he took office as 44th president in the depths of the Great Recession and warned the nation of hard times ahead.

This time, the president challenged the nation – both about what it ought to be and what it ought to do.

“My fellow Americans, we are made for this moment,” he said, “and we’ll seize it – so long as we seize it together.”

Above all, he said, America must remain a land of opportunity.

“Our country cannot succeed when a shrinking few do very well and a growing many barely make it,” he said. “We believe that America’s prosperity must rest upon the broad shoulders of a rising middle class.”

Two and a half months after winning an election against Republican Mitt Romney in which the GOP sometimes divided the nation into “makers” and “takers,” Obama turned the party’s rhetoric against it to defend the entitlements that the opposition party wants to curtail.

“The commitments we make to each other through Medicare and Medicaid and Social Security, these things do not sap our initiative; they strengthen us,” Obama said. “They do not make us a nation of takers; they free us to take the risks that make this country great.”

Obama’s 18-minute speech highlighted a day of ceremony and celebration. After attending a church service and traveling to the Capitol for the Inauguration, the second-term president and first lady Michelle Obama led a parade back to the White House before preparing to attend two black-tie galas in his honor.

A crowd estimated at 700,000 – less than half the size of four years ago – turned out for the Inauguration, a ceremonial event this year because the official oath was take Sunday, the constitutionally required date of Jan. 20. When that date falls on a Sunday, tradition calls for the public celebration to take place the next day.

“It is a good moment to rejoice today at this 57th presidential inaugural ceremony, and it is the perfect moment to renew our collective faith in the future of America,” said Sen. Charles E. Schumer, D-N.Y., chairman of the organizing committee and master of ceremonies.

After paying little attention to global warming in his first term, Obama devoted two full paragraphs of his speech to the issue, in part: “We will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations. Some may still deny the overwhelming judgment of science, but none can avoid the devastating impact of raging fires and crippling drought and more powerful storms.”

Similarly, Obama called for action on immigration reform.

“Our journey is not complete until we find a better way to welcome the striving, hopeful immigrants who still see America as a land of opportunity – until bright young students and engineers are enlisted in our workforce rather than expelled from our country,” he said.

In the same passage of the speech, Obama became the first president ever to call for gay rights – or even use the term “gay” – from the podium at an Inauguration. “Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law, for if we are truly created equal, then surely the love we commit to one another must be equal as well,” he said.

In the same vein, Obama said the American journey will not be complete until women receive equal pay for equal work, until citizens no longer have to wait in line for hours to vote and “until all our children, from the streets of Detroit to the hills of Appalachia, to the quiet lanes of Newtown, know that they are cared for and cherished and always safe from harm.”

Obama’s call for a broader vision of American rights prompted roar after roar from the crowd that stretched westward from the Capitol to the Washington Monument.

But the cry of dissent – and depth of division in the nation – could be heard in the voice of a man who climbed a tree on the National Mall and spent the entire ceremony protesting abortion. “Think of the blood of the babies!” he shouted, according to the BuzzFeed website. “Democrats are baby-killers! What about the babies! Stop Obama!”

Abortion is just one of the issues dividing Obama and Republicans.

Fresh from a battle over the “fiscal cliff” that resulted in hard-won tax increases on the wealthy, Obama implicitly challenged Republicans again over their demands for budget cuts.

“We must make the hard choices to reduce the cost of health care and the size of our deficit,” he said. “But we reject the belief that America must choose between caring for the generation that built this country and investing in the generation that will build its future.”

Despite the barely disguised criticism of Republicans that seemed to be laced through the speech, Obama’s address prompted words from Senate Minority Leader Mitch McConnell, R-Ky.

“The president’s second term represents a fresh start when it comes to dealing with the great challenges of our day; particularly, the transcendent challenge of unsustainable federal spending and debt,” McConnell said in a statement. “Republicans are eager to work with the president on achieving this common goal, and we firmly believe that divided government provides the perfect opportunity to do so.”

At a luncheon in the Capitol’s Statuary Hall after the ceremony, Obama struck a collegial tone in his address to top lawmakers, Supreme Court justices, Cabinet officials and top diplomats. “The longer you are there [in the White House], the more humble you become, and the more mindful you are that it is beyond your powers individually to move this great country,” Obama said.

Acknowledging “profound differences” among the leaders in the room, Obama nevertheless said, both at the luncheon and in his earlier address to the nation, that he’s confident those differences can be bridged.

“Decisions are upon us, and we cannot afford delay,” he said at the Inauguration. “We cannot mistake absolutism for principle, or substitute spectacle for politics, or treat name-calling as reasoned debate.

“We must act, knowing that our work will be imperfect. We must act, knowing that today’s victories will be only partial and that it will be up to those who stand here in four years and 40 years and 400 years hence to advance the timeless spirit once conferred to us in a spare Philadelphia hall.”

News wire services contributed to this report.