WASHINGTON — 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 today, 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 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.

He previewed an ambitious second-term agenda, devoting several sentences 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.

Looking to the challenges ahead, 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 the Rev. Martin Luther King Jr. 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 sworn in for his second term as the nation's second in command.

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, the same day the nation marked the late civil rights leader King's birthday.

Following his address, Obama headed into the Capitol for a lunch with lawmakers and to sign nominations for several Cabinet members. Then it was off for the traditional inaugural parade down Pennsylvania Avenue and two glitzy evening inaugural balls.

After a stunning sunrise, the weather for the swearing-in and parade was chilly — upper 30s rising into the lower 40s — and overcast.

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.

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 earlygoers 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.