(ABC NEWS) -- Invoking the nation's founding values, President Obama marked the start of his second term with a sweeping call for "collective action" to confront the economic and social challenges of America's present and future.
"That is our generation's task, to make these words, these rights, these values -- of life, and liberty, and the pursuit of happiness -- real for every American," Obama said in an inaugural address delivered from the west front of the U.S. Capitol.
"Being true to our founding documents does not require us to agree on every contour of life; it does not mean we will all define liberty in exactly the same way, or follow the same precise path to happiness. Progress does not compel us to settle centuries-long debates about the role of government for all time," he said, giving nod to the yawning partisan divide.
"But it does require us to act in our time."
The call to action, on the eve of what's shaping up to be another contentious term with Republicans and Congress, aimed to reset the tone of debate in Washington and turn the page on the political battles of the past.
"For now decisions are upon us, and we cannot afford delay. We cannot mistake absolutism for principle, or substitute spectacle for politics, or treat name-calling as reasoned debate," Obama said. "We must act, knowing that our work will be imperfect."
The address, lasting a little less than 20 minutes, laid out in broad terms Obama's vision for the next four years, alluding to looming policy debates on the war in Afghanistan, deficit reduction, immigration, and overhaul of Social Security and Medicare.
Obama also became the first president, at least in recent inaugural history, to make explicit mention of equality for gay and lesbian Americans. He made repeated mentions of "climate change," something no president has said from such a platform before.
The president stuck closely to his campaign themes, offering few new details of his policy proposals, however. Those are expected to come next month in the State of the Union address Feb. 12.
"A decade of war is now ending. An economic recovery has begun. America's possibilities are limitless, for we possess all the qualities that this world without boundaries demands: youth and drive; diversity and openness; an endless capacity for risk and a gift for reinvention," Obama said, sounding optimistic tones.
"We are made for this moment, and we will seize it," he said, "so long as we seize it together."
Hundreds of thousands packed the National Mall in chilly 40-degree temperatures and brisk wind to hear Obama's remarks and witness the ceremonial swearing-in. While the crowds were smaller than four years ago, the U.S. Park Police said the Mall reached capacity and was closed shortly before Obama took the podium.
Shortly before the address, Obama placed his left hand on the stacked personal Bibles belonging to President Abraham Lincoln and Dr. Martin Luther King Jr., and raised his right to repeat the oath administered by Supreme Court Chief Justice John Roberts.
"I, Barack Hussein Obama, do solemnly swear that I will faithfully execute the office of president of the United States," he said, "and will to the best of my ability, preserve, protect and defend the Constitution of the United States."
Obama and Biden were both officially sworn in during private ceremonies Sunday, Jan. 20, the date mandated by the Constitution for presidents to begin their terms.
Top Republicans publicly congratulated the president on his inauguration and urged Americans to celebrate the history and symbolism of the day.
"Today we praise the American tradition of transferring or reaffirming immense power in the inauguration of the president of the United States. We do this in a peaceful, orderly way," Republican Sen. Lamar Alexander of Tennessee said.
"Find the good and praise it."
House Majority Leader Eric Cantor, a Republican from Virginia, tweeted congratulations to Obama and Biden.
Senate Minority Leader Mitch McConnell of Kentucky said in a statement that he wishes the president "well in the fulfillment of his duty to lead the U.S. at home and abroad over the next four years."
"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.
"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."
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