Washington - Barack Hussein Obama ceremonially opened his second term Monday with an assertive inaugural address that offered a robust articulation of modern liberalism in America, arguing that "preserving our individual freedoms ultimately requires collective action."

On a day that echoed with refrains from the civil rights era and tributes to the Rev. Martin Luther King Jr., Obama dispensed with the post-partisan appeals of four years ago to lay out a forceful vision of advancing gay rights, showing more tolerance toward illegal immigrants, preserving the social welfare safety net and acting to stop climate change.

At times he used his speech, delivered from the West Front of the Capitol, to reprise arguments from the fall campaign, rebutting the notion expressed by conservative opponents that America risks becoming "a nation of takers" and extolling the value of proactive government in society. Instead of declaring the end of "petty grievance," as he did taking the oath as the 44th president in 2009, he challenged Republicans to step back from their staunch opposition to his agenda.

"Progress does not compel us to settle centuries-old debates about the role of government for all time - but it does require us to act in our time," he said in the 18-minute address. "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. We must act."

Obama used Abraham Lincoln's Bible, as he did four years ago, but this time added King's Bible as well to mark the holiday honoring the civil rights leader. He became the first president to mention the word "gay" in an inaugural address as he equated the drive for same-sex marriage to the quest for racial and gender equality.

The festivities at the Capitol came a day after Obama officially took the oath in a quiet ceremony with his family at the White House on the date set by the Constitution. With Inauguration Day falling on a Sunday, the swearing-in was then repeated for an energized mass audience a day later, accompanied by the pomp and parade that typically surrounds the quadrennial tradition.

Hundreds of thousands of people gathered on a brisk but bright day, a huge crowd by any measure, although far less than the record turnout four years ago. If the day felt restrained compared with the historic mood the last time, it reflected a more restrained moment in the life of the country. The hopes and expectations that loomed so large with Obama's taking the office in 2009, even amid economic crisis, have long since faded into a starker sense of the limits of his presidency.

Now 51 and noticeably grayer, Obama appeared alternately upbeat and reflective. When he re-entered the Capitol at the conclusion of the ceremony, he suddenly stopped his entourage to turn back toward the cheering crowds gathered on the National Mall.

"I want to take a look one more time," he said. "I'm not going to see this again."

If the president was wistful, he was firm in his message. He largely eschewed foreign policy except to recommend engagement over war, instead focusing on addressing poverty and injustice at home. He did little to adopt the language of the opposition, as he has done at moments in the past, and instead directly confronted conservative philosophy.

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

The phrase "nation of takers" was a direct rebuke to Republicans like Rep. Paul Ryan of Wisconsin, last year's vice presidential nominee, and several opposition lawmakers took umbrage at the president's tone.

"I would have liked to see a little more on outreach and working together," said Sen. John McCain of Arizona, the Republican who lost to Obama four years ago. "There was not, as I've seen in other inaugural speeches, 'I want to work with my colleagues."'

Rep. Pete Sessions of Texas, a member of the Republican leadership, said that from the opening prayer to the closing benediction, "It was apparent our country's in chaos and what our great president has brought us is upheaval." He added, "We're now managing America's demise, not America's great future."

Obama struck a more conciliatory note during an unscripted toast during lunch with congressional leaders in Statuary Hall after the ceremony.

"Regardless of our political persuasions and perspectives, I know that all of us serve because we believe that we can make America for future generations," he said.

For the nation's 57th presidential inauguration, a broad section of downtown Washington was off limits to vehicles and some bridges across the Potomac River were closed to regular traffic, as military Humvees were stationed at strategic locations around the city.

Joining the president through the long day were the first lady, Michelle Obama, and their daughters, Malia, 14, and Sasha, 11. The young girls were playful. Malia at one point sneaked up behind her father and cried out, "Boo!" Sasha used a smartphone to take a picture of her parents kissing in the reviewing stand, then made them do it again. Both girls bounced with the martial music at the Capitol.

Obama's day began with a service at St. John's Church, across Lafayette Square from the White House, where the Rev. Andy Stanley told him to "leverage that power for the benefit of other people in the room." At the Capitol, Myrlie Evers-Williams, the civil rights leader, delivered the invocation and the Brooklyn Tabernacle Choir performed the "Battle Hymn of the Republic."

Obama was more specific in discussing policy than presidents typically are in an Inaugural Address. Particularly noticeable was his recommitment to fighting climate change. "We will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations," he said.

He made no direct mention of terrorism, the issue that has so consumed the nation for the past decade, but offered a more inward-looking approach to foreign policy, saying that "enduring security and lasting peace do not require perpetual war." He also talked of overhauling immigration rules so "bright young students and engineers are enlisted in our workforce, rather than expelled from our country."

For a president who opposed same-sex marriage as recently as nine months ago, the speech was a clear call for gay rights, as he noted the journey "through Seneca Falls and Selma and Stonewall," symbolically linking seminal moments in the struggles for equal rights for women, blacks and gays and lesbians.

"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.

The expanse between the Capitol and the Washington Monument was filled with supporters, many of them blacks attending only the second inauguration of a black president. As large TV screens flickered in and out and the audio often warbled, the ceremony was difficult to follow for many braving the Washington chill.

The speech was followed by song, poem and benediction from Kelly Clarkson, Richard Blanco, the Rev. Luis Leon and Beyonce. The president and first lady got out of their motorcade twice to walk stretches along Pennsylvania Avenue. Biden and Jill Biden did as well, and the vice president greeted bystanders with gusto.

The two families then settled into the specially built bulletproof reviewing stand to watch the parade. Obama, who often uses Nicorette to tame an old smoking habit, was spotted chewing as the bands marched past.

By evening, the Obamas were heading out to celebrate, planning to attend two official inaugural balls, down from the 10 four years ago, before returning home for the next phase of their sojourn in the White House.