Obama didn't lose sight of the import of the moment, repeating themes he voiced during his presidential bid and honoring the struggles of previous generations that enabled him to become the United States' commander in chief.
"On this day, we gather because we have chosen hope over fear, unity of purpose over conflict and discord," Obama told the in-person and broadcast audiences, proclaiming later, "The time has come to reaffirm our enduring spirit; to choose our better history; to carry forward that precious gift, that noble idea, passed on from generation to generation: the God-given promise that all are equal, all are free, and all deserve a chance to pursue their full measure of happiness."
His day began with a 45-second car ride from Blair House to St. John's Episcopal Church, where Obama and his wife Michelle met running mate Joe Biden and his wife, Jill, for the traditional inauguration day service.
Meanwhile, the National Mall filled with people as an estimated 1 million to 2 million people were expected to witness Obama's swearing in. Fashion statements were scrapped for ear muffs, woolen caps, gloves and heavy coats as temperatures were in the 20s, but brisk conditions made it feel more like 10 degrees.
"The world is watching today," Sen. Dianne Feinstein, chairwoman of the Joint Congressional Committee on Inaugural Ceremonies, said. "We gather to etch another line in the solid stone of history."
While Obama took the oath of office after noon, he already was president because of a constitutional provision directing that the president-elect becomes president at noon, Jan. 20.
Obama, 47, gave his full name -- Barack Hussein Obama -- when repeating the oath administered by U.S. Supreme Court Chief Justice John G. Roberts Jr., who muffed his lines by misplacing the word "faithfully."
Both he and Obama recovered, with Roberts being the first to recognize him as "Mr. President."
As Obama basked in adulation, George and Laura Bush bade farewell to city they called home for eight sometimes-rocky years.
At an Andrews Air Force Base hangar, Bush told about 2,000 aides, supporters and their families he was "thankful, grateful and ... joyful" for the opportunity to be president, adding that his administration "led with conviction."
The energy in what should have been the power lunch of all power lunches with congressional leaders, former presidents and vice presidents, and Cabinet nominees was dampened when Sen. Ted Kennedy, D-Mass., suffered a seizure doctors later said was brought on by fatigue. Kennedy, D-Mass., who has been battling brain cancer, was removed from the Capitol by stretcher and hospitalized overnight.
In remarks after he and Biden received gifts and a commemorative photograph, Obama said, "I would be lying to you if I did not say that right now a part of me is with him (Kennedy)."
Sen. Robert Byrd, D-W.Va., the oldest member of the Senate, reportedly was so distressed, he was removed from the luncheon in a wheelchair.
By the time the Obamas and first daughters Malia and Sasha reached the reviewing stand for the inaugural parade, night began to fall on the nation's capital. The Obamas and the Bidens kept with the tradition of walking part of the parade route, with cheers erupting from the sardine-packed crowd lining the route when the Obamas emerged from their fortified vehicle.
With a look both to the present and the future, Obama ended his inaugural address by saying, "Let it be said by our children's children that when we were tested we refused to let this journey end, that we did not turn back nor did we falter; and with eyes fixed on the horizon and God's grace upon us, we carried forth that great gift of freedom and delivered it safely to future generations."