Standing on the West Front of the Capitol as the first African-American ever sworn in as president, Obama celebrated that historic achievement, noting that "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."
In moments of crisis, "America has carried on not simply because of the skill or vision of those in high office, but because we the people have remained faithful to the ideals of our forbears and true to our founding documents," Obama said.
"So it has been. So it must be with this generation of Americans."
Evoking the names and values of the Founding Fathers is commonplace in presidential speeches, but in Obama's case the device seemed intended to make a larger point:
The change he hopes to bring about will require even his supporters to accept things they don't want to accept, work with opponents they've long demonized and break long-ingrained lifestyles.
Americans as a whole must adopt a new, more self-denying way of life with little room for "those who prefer leisure over work, or seek only the pleasures of riches and fame," he said. "What is required of us now is a new era of responsibility."
In a passage that echoed Franklin Roosevelt's first inaugural, Obama said, "Our workers are no less productive than when this crisis began. Our minds are no less inventive, our goods and services no less needed than they were last week or last month or last year. Our capacity remains undiminished.
"But our time of standing pat, of protecting narrow interests and putting off unpleasant decisions — that time has surely passed. Starting today, we must pick ourselves up, dust ourselves off and begin again the work of remaking America."
If the speech was exceptionally somber and included relatively few lines designed to draw roars of approval from the enormous crowd, the day nonetheless resounded with jubilation.
More than 1 million people flocked to the National Mall to take part in the event, spilling outward from the gleaming white Capitol steps toward the Washington Monument and the Lincoln Memorial more than a mile away.
Choirs sang. The world's finest musicians — including classical violinist Itzhak Perlman and Yo-Yo Ma, the cellist, along with soul singer Aretha Franklin — performed. High school bands paraded. And tears streamed down faces, weathered and smooth alike, here and around the globe, as the son of a white American and a black African immigrant ascended to his place in history.
The only shadow on the day was cast during the luncheon for the new president in the Capitol's Statuary Hall hosted by House and Senate leaders: Sen. Edward Kennedy (D-Mass.) suffered a seizure and was taken to a hospital. When Kennedy was stricken, Obama hurried to his side, later recalling that Kennedy had been in the Senate for the passage of the Voting Rights Act in 1965.
Kennedy suffers from brain cancer, but an aide said he was awake, talking with family members and feeling well.
Sen. Robert Byrd (D-W.Va.), the oldest member of the Senate, was sitting near Kennedy and became visibly upset. He was taken from the lunch but an aide said later he is fine.
As is traditional, President George W. Bush and Laura Bush were whisked away by helicopter immediately after the inaugural ceremony and headed for their home state of Texas after a private farewell to staff at nearby Andrews Air Force Base.
And almost at once, the wheels of the new administration began to turn.
In the afternoon, new White House Chief of Staff Rahm Emanuel, as expected, put a hold on all regulations the Bush administration had been drafting, pending a review by the new team. Obama is expected to begin issuing his own administrative measures this week.
Obama made the appointment of his Cabinet his first official act, and the Senate approved six secretaries and a budget director before the day was over, though Senate Republicans delayed others.
Secretary of State-designate Hillary Clinton had been expected to win approval Tuesday, but Sen. John Cornyn (R-Texas) held her nomination up for a day to permit further questioning about her husband and his charitable foundation's foreign donors.
Others who face a more protracted process include Labor nominee Hilda Solis, Treasury nominee Timothy Geithner, attorney general nominee Eric Holder and Transportation nominee Ray LaHood.
The delays are expected to be temporary.
Obama also still needs to nominate a commerce secretary pick to replace Gov. Bill Richardson, who withdrew from consideration in the midst of a scandal in his home state of New Mexico.
The finality of the transfer of power was signaled in small ways and large. A picture of Bush vanished from the White House Web site shortly after noon, and Obama's portrait appeared in its place.
That Obama was taking office in challenging times, both domestic and foreign, he was quick to acknowledge, including an economic crisis as ominous as any since Franklin Roosevelt moved into the White House amid the Great Depression.
"Our nation is at war, against a far-reaching network of violence and hatred," Obama said. "Our economy is badly weakened, a consequence of greed and irresponsibility on the part of some, but also our collective failure to make hard choices and prepare the nation for a new age."
The way out of the domestic morass, Obama said, will require a more active role for government.
Indirectly rejecting Bill Clinton's assertion in 1996 that the era of big government was over, Obama said, "The question we ask today is not whether our government is too big or too small, but whether it works — whether it helps families find jobs at a decent wage, care they can afford, a retirement that is dignified."
On foreign policy, Obama vowed to outlast and ultimately defeat terrorists.
But he went out of his way to extend his hand to the Muslim world.
"To the Muslim world, we seek a new way forward, based on mutual interest and mutual respect," he said.
He also declared that the United States would once more play the role of world leader. "We will begin to responsibly leave Iraq to its people, and forge a hard-earned peace in Afghanistan," he declared. "With old friends and former foes, we will work tirelessly to lessen the nuclear threat, and roll back the specter of a warming planet.
"Today I say to you that the challenges we face are real. They are serious and they are many. They will not be met easily or in a short span of time. But know this, America — they will be met."
Inauguration Day dawned with hundreds of thousands of people assembling in early-morning darkness to watch the day's events unfold — and to cheer, sing and tell stories.
In the crowd, there was sustained booing of Bush at some points in the program.
At the congressional lunch that followed the Bushes' departure by helicopter, Obama worked the room like a bridegroom at a wedding. When Kennedy was taken ill, Obama reminded the crowd in a halting voice that Kennedy had long been a supporter of equal justice.
"So I would be lying to you if I did not say that right now a part of me is with him," Obama said.
At another point, Michelle Obama joined her husband to review the troops on the steps of the Capitol. When Obama went to shake the hand of the military officer at his side, the president appeared startled when he got a salute instead as the new commander in chief.
But Obama and the new first lady were all smiles and ease as they walked a section of their parade route, the silver collar of Michelle Obama's yellow-gold dress glinting in the afternoon sun. In the evening, the Obamas were to attend and speak at 10 official inaugural balls.
Yet it was the words of the afternoon that resonated beyond their delivery.
The crowd, mesmerized, listened on the back of the West Lawn as the speech rolled across the Mall from a sound system that took two or three seconds to get to the farthest reaches of the crowd.
The echo meant that the field was never quiet, even when Obama paused, as though the words of the day couldn't be contained in a single moment or place.