WASHINGTON (AP) — Declaring "our journey is not complete," President Barack Obama took the oath of office for his second term before a crowd of hundreds of thousands Monday, urging the nation to set an unwavering course toward prosperity and freedom for all its citizens and protect the social safety net that has sheltered the poor, elderly and needy.

"Our country cannot succeed when a shrinking few do very well and a growing many barely make it," Obama said in his relatively brief, 18-minute address. "We believe that America's prosperity must rest upon the broad shoulders of a rising middle class," he added, echoing his calls from the presidential campaign that catapulted him to re-election.

The president declared that a decade of war is ending, as is the economic recession that consumed much of his first term.

The inaugural fanfare spread across the capital Monday, including a traditional lunch with lawmakers on Capitol Hill. Before departing the Capitol, Obama paused in the Rotunda in front of a bust of Martin Luther King Jr., the slain civil rights leader whose birthday holiday coincided with the inaugural festivities.

From the Capitol, the president and first lady Michelle Obama climbed into the black armored limousine that ferried them past cheering crowds lining Pennsylvania Avenue for the inaugural parade. Surrounded by Secret Service agents, the Obamas stepped out of the limousine and walked the parade route for about eight minutes, holding hands and waving to the exuberant crowds.

Before diving into the afternoon celebrations, Obama previewed an ambitious second-term agenda, devoting several sentences in his address to the threat of global climate change and saying that failure to confront it "would betray our children and future generations." Obama's focus on climate change was notable given that he barely dealt with the issue in his first term.

In an era of looming budget cuts, he said the nation has a commitment to costly programs such as Medicare, Medicaid and Social Security. "These things do not sap our initiative, they strengthen us," he said.

Sandwiched between the bruising presidential campaign and relentless fiscal fights, Monday's inaugural celebrations marked a brief respite from the partisan gridlock that has consumed the past two years. Perhaps seeking a fresh start, Obama invited several lawmakers to the White House for coffee before his speech, including the Republican leaders with whom he has frequently been at odds.

Among then was the Senate's top Republican, Mitch McConnell of Kentucky. In a statement following Obama's swearing-in, McConnell said the president's second term represents "a fresh start when it comes to dealing with the great challenges of our day."

Looking ahead to those challenges, Obama implored Congress to find common ground over the next four years. And seeking to build on the public support that catapulted him to the White House twice, the president said the public has "the obligation to shape the debates of our time."

"Not only with the votes we cast, but with the voices we lift in defense of our most ancient values and enduring ideals," Obama said.

Moments earlier, Obama placed his hand on two Bibles — one used by King and the other by Abraham Lincoln — and recited the brief oath of office. Michelle Obama held the Bibles, one on top of the other, as daughters Malia and Sasha looked on.

Vice President Joe Biden was also sworn in for his second term as the nation's second in command. Former Presidents Bill Clinton and Jimmy Carter, several Cabinet secretaries and dozens of lawmakers were on hand to bear witness to history.

Monday's oaths were purely ceremonial. The Constitution stipulates that presidents begin their new terms at noon on Jan. 20, and in keeping with that requirement, Obama was sworn in Sunday in a small ceremony at the White House. Because inaugural celebrations are historically not held on Sundays, organizers pushed the public events to Monday.

Obama soaked in the history on a day full of traditions as old as the Republic. Gazing over the crowd before retreating into the Capitol, he said, "I want to take a look, one more time. I'm not going to see this again."

After a stunning sunrise, the weather for the swearing-in and parade was chilly — upper 30s rising into the lower 40s. Overcast skies gave way to bright sunshine for the parade.

Once the celebrations subside, Obama will be confronted with an array of pressing priorities: an economy still struggling to fully a recover, the fiscal fights with a divided Congress, and new threats of terrorism in North Africa. The president has also pledged to tackle immigration reform and stricter gun laws in the wake of the school shootings in Newtown, Conn., — sweeping domestic reforms that will require help from reluctant lawmakers.

Obama is also facing fresh concerns about terrorism in North Africa. In the midst of the inaugural celebrations, a U.S. official said two more Americans died in Algeria, bringing the U.S. death toll from a four-day siege at a natural gas plant to three. Seven Americans survived, the official said.

The president did not offer any specific prescriptions for addressing the challenges ahead, though he is expected to offer more detail in his Feb. 12 State of the Union address.

Asserting "America's possibilities are limitless," he declared at the Capitol: "My fellow Americans, we are made for this moment, and we will seize it, so long as we seize it together."

"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."

Obama's second inaugural lacked the electric enthusiasm of his first, when 1.8 million people crammed onto the National Mall to witness the swearing-in of the nation's first black president. Far fewer people attended this year's inauguration — officials estimated up to 700,000 people — but the crowd still stretched from the Capitol to the Washington Monument. And shortly before the president spoke, U.S. Park Police announced that the public viewing areas on the Mall were full.

Security was tight across Washington, with streets closed off for blocks around the White House and Capitol Hill. Military Humvees and city buses were being used to block intersections. Volunteers fanned out near the Mall to help direct the crowds.

David Richardson of Atlanta and his two young children were among the early-goers who headed to the Mall before sunrise.

"We wanted to see history, I think, and also for the children to witness that anything is possible through hard work," Richardson said.

Wendy Davis of Rome, Ga., was one of thousands of inaugural attendees who packed Metro trains. Davis came four years ago as well but was among the many ticketholders who couldn't get in then because of the massive crowds.

"I thought I was early last time, but I obviously wasn't early enough," she said.

By 8 a.m. thousands of people were also waiting in security lines that stretched a block to gain access to the spots along the parade route that were accessible to the general public without a special ticket.

The cold weather was easily tolerated by Marie-France Lemaine of Montreal, who received the trip to the inaugural as a birthday present from her husband. She headed up an Obama advocacy group in Quebec that cheered on the president from north of the border.

"The American president affects the rest of the world," she said.

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Associated Press writers Darlene Superville, Matt Barakat, Alan Fram, Donna Cassata, Jim Kuhnhenn, Mary Clare Jalonick and Nancy Benac contributed to this report.

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Follow Julie Pace at http://twitter.com/jpaceDC

President Barack Obama discussed the need to act on climate change in his inauguration address, highlighting previously stated intentions to make the issue a priority in his second term.

In his speech, Obama tied failure to respond to climate change with a betrayal of 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," he said today (Jan. 21). "The path towards sustainable energy sources will be long and sometimes difficult. But America cannot resist this transition; we must lead it."

Obama went on to cast green technology in a positive light, arguing that America "must claim its promise." The message resonated with climate scientists and environmental groups.

"I was reassured to see him reaffirm that facts matter, and that the science overwhelmingly indicates that climate change is not only real, but is already posing a serious threat to society," Michael Mann, a climate scientist at Pennsylvania State University, told LiveScience. However, Mann said, the details of Obama's climate policy still need to be fleshed out. [The Reality of Climate Change: 10 Myths Busted]

Climate at the inauguration

Obama's mention of the c-word is not the first time climate has made an appearance in an inaugural speech, but presidents rarely use the occasion to discuss the environment. In 2010, Obama made a passing reference to rolling back "the specter of a warming planet." Former President Bill Clinton made one reference apiece to the need for a clean environment in his 1993 and 1997 inaugural speeches, but did not specifically mention global warming.

This year's reference was much more extended than that in Obama's first-term speech.

"We, the people, still believe that our obligations as Americans are not just to ourselves, but to all posterity," Obama said. "We will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations."

Will there be action?

Environmental groups praised Obama's words and used the opportunity to push for policy action.

"He did a good job laying out a narrative about climate change and why we should all care about taking meaningful action," said Travis Franck, a policy analyst for the nongovernmental organization Climate Interactive. "It will affect our children and grandchildren in many ways: their economic opportunities, their health, their safety from disasters, their recreation activities, and their sense of pride in America and its place in the world."

But the action Obama intends to take remains unknown. Climate experts say that sweeping policies are unlikely to make it through Congress in Obama's second term. The president does have some leeway to make regulatory changes via executive action.

"It would have been much more encouraging if he went on to say that he would put the power of his office into the fight to get the country onto a sustainable energy path, by working for regulations and legislation that would incentivize energy efficiency and the transition to clean energy and make the burning of fossil fuels less attractive," Elizabeth Sawin, the co-director of Climate Interactive, told LiveScience.

"Citizens who care about clean energy and a livable planet are going to need to continue to organize and pressure elected officials, including the president, if we want to see concrete action and climate-protecting policy coming from Washington," Sawin said.