Barack Hussein Obama became the nation’s 44th president Tuesday, telling a crowd that stretched from the steps of the Capitol to the Lincoln Memorial, and a watching nation, that “we must pick ourselves up, dust ourselves off and begin again the work of remaking America.”

Chuck Kennedy/AP Photo/Pool Barack Obama, left, takes the oath of office from Chief Justice John Roberts, not seen, as his wife Michelle, holds the Lincoln Bible and daughters Sasha, right and Malia, watch at the U.S. Capitol in Washington, Tuesday.

The ascent of the country’s first African American president, and the peaceful transfer of power in tumultuous times both at home and abroad, drew an enormous mass of well-wishers and witnesses to history. An early estimate from a senior security official put the number of people on the Mall at 2 million.

Obama outlined the challenges facing the country: a collapsing economy, wars on two fronts, a lack of confidence in government and enemies who hate the very way of American life.

They can be addressed, he said, with “a new era of responsibility — a recognition, on the part of every American, that we have duties to ourselves, our nation, and the world, duties that we do not grudgingly accept but rather seize gladly, firm in the knowledge that there is nothing so satisfying to the spirit, so defining of our character, than giving our all to a difficult task.

“This is the source of our confidence — the knowledge that God calls on us to shape an uncertain destiny.”

The birth of a new administration always marks a burst of American excitement and anticipation, and in Obama’s case, a surge of optimism in a country beset by troubles. But perhaps no inauguration in recent times has matched the anticipation attending the senator from Illinois, who made “hope” and “change” the bywords of his campaign.

Obama was accompanied to the West Front of the Capitol by President George W. Bush and sworn in by Chief Justice John G. Roberts Jr. Obama took the oath by stating his full name, the one he said opponents once used to try to make him seem apart from mainstream America.

It was the first time the chief justice administered the oath — indeed, the first time any chief justice had sworn in a man who voted against his confirmation — and both men stumbled over the words. But the sight of the two youthful leaders — Roberts, 53, the second-youngest chief justice, and Obama, 47, the fourth-youngest man elected president — underscored the theme of generational change.

So did the presence of the youthful Michelle Obama and the couple’s two grade-school daughters, Malia and Sasha, dressed in candy tones of blue and pink.

Continuity was marked by the swearing-in of former Delaware senator Joseph R. Biden Jr. as vice president, the oath administered by 88-year-old Justice John Paul Stevens, a Republican appointee who is now the leader of the court’s liberal contingent and the second-oldest man to serve on the court.

Obama laid his hand on the burgundy-velvet-covered Bible that was used by Abraham Lincoln in 1861, and history again trembled. The chief justice that day was Marylander Roger Brooke Taney, the author of the Dred Scott decision that said blacks could never be citizens. The Constitution, Taney said, recognized blacks as “beings of an inferior order, and altogether unfit to associate with the white race, either in social or political relations.”

Obama called it the “meaning of our liberty and our creed” that those days are no more: “Why men and women and children of every race and every faith can join in celebration across this magnificent Mall, and why a man whose father less than 60 years ago might not have been served at a local restaurant can now stand before you to take a most sacred oath.”

It was his most overt reference to the historical significance of his achievement.

History, history: the word was on the lips of nearly everyone who rode buses through the night or crowded Metro platforms in pre-dawn darkness or walked through crowded streets just to get to a point where, with luck, the new president would be a speck on the horizon.

Ellamae Simmons, 90, was hoisted out of her wheelchair by two young family members, who carried her down the escalator at the Farragut North Metro station at 6:30 a.m. The first train to arrive was too crowded for the wheelchair, but Simmons, a retired doctor who flew in from San Francisco on Saturday, did not complain about waiting for a later train.

“I would have gone across the water to see Mr. Obama,” Simmons said as she boarded.

The day marked the end of an extraordinary journey to the White House. Obama becomes perhaps the country’s most improbable president, the son of a Kenyan scholar and a white Kansas mother, raised in the exotic climes of Hawaii and Indonesia and bearing what he describes as a “funny name.”

Just five years ago, he was a middle-of-the-pack Illinois state senator in Springfield, and now fills the nation’s highest office even before finishing his first term in the United States Senate.

But his candidacy inspired young voters and an unprecedented outpouring of African American voters, and his motivational message drew independents and disaffected voters of both parties.

Bishop T.D. Jakes drew from the campaign for his sermon at the morning church service at St. John’s Episcopal Church, where the Obamas joined Biden and his wife, Jill Biden.

“The problems are mighty and the solutions are not simple and everywhere you turn there will be a critic waiting to attack every decision that you make,” Jakes said. Then came a message borne of inspiration and campaign rhetoric: “But you are all fired up, sir, and you are ready to go. And this nation goes with you. God goes with you.”

The Obamas arrived at the White House at 9:52 a.m., and were greeted three minutes later by the Bushes. Michele Obama gave Laura Bush a white package tied with red ribbon, and the two couples embraced and posed for photographers. Meeting for coffee is a tradition for the outgoing and incoming presidents before making the trip to Capitol Hill.

The Rev. Rick Warren, who delivered the invocation at the inaugural ceremony, said, “We celebrate a hinge point in history,” and he prayed for God to give Obama “the wisdom to lead us with humility, the courage to lead us with integrity, the compassion to lead us with generosity.”

Obama had begun the process for his much-anticipated inaugural speech shortly before Thanksgiving, laying out the broad outlines of what he hoped to say, senior transition officials said. In particular, Obama told his speechwriters he wanted to do what some of the best inaugural speeches in history have done: Explain to the American people “the moment that we find ourselves in right now,” one official said.