With a call to unite to confront the greatest challenges facing the country in generations, Barack Obama was sworn in as the 44th president of the United States, the final step in a historic run from relative national obscurity fours years ago to become the first African-American to occupy the White House.
The Illinois Democrat, whose vow to reinvent Washington challenged long-entrenched interests, adrenalized an out-of-power political party, and inspired millions who had never before voted, took the oath of office just before noon on a cold, overcast day. The Capitol dome looming behind him, Obama pledged to lead the country on the difficult path back to peace and prosperity.
"Today I say to you that the challenges we face are real," Obama said, according to prepared remarks. "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."
But along with his campaign theme of change, the new president also called for unification and a "new era of responsibility."
The challenges are great. As he assumes the Oval Office, Obama faces deeper problems than any president since World War II - a complicated brew of economic and national security challenges in a swiftly changing world. In the last year alone, the U.S. economy has shed 2.6 million jobs; Wall Street titans have crumbled; and confidence in the U.S. financial system has evaporated as workers and retirees watch their savings disappear.
Meanwhile, a series of intractable foreign-policy issues also will occupy the new president, from the conflict in Gaza to Pakistan and the Guantanamo Bay detention facility, which Obama has promised to close.
Noting that the economy is "badly weakened," Obama said "the state of the economy calls for action, bold and swift, and we will act - not only to create new jobs, but to lay a new foundation for growth."
Dismissing critics of his massive stimulus plan, Obama said the question isn't whether government is "too big or too small, but whether it works."
He also cautioned against anti-capitalist sentiment, saying that market shouldn't be seen as either a "force for good or ill."
"Its power to generate wealth and expand freedom is unmatched, but this crisis has reminded us that without a watchful eye, the market can spin out of control - and that a nation cannot prosper long when it favors only the prosperous," Obama said.
His agenda is ambitious. The first order of business is a budget-busting $825 billion economic stimulus package designed to save or create three million to four million jobs. This is followed by plans to exit Iraq and focus on the fight against extremists in Afghanistan; a long-term fix of the health-care system; and an overhaul of the complex rules governing the financial system.
All that will be addressed against a stark backdrop. Most Americans expect the current recession to linger for at least another year, while the budget deficit - already in record territory last year - is projected to blast through the $1 trillion mark.
"I don't think a president has taken office facing a more severe and immediate set of problems since (Franklin D.) Roosevelt," said Brian Balogh, a historian at the University of Virginia's Miller Center of Public Affairs.
Obama's transition was more eventful than most. The president-elect quickly put together a cabinet; sold his stimulus ideas on the economy to the public; and lobbied lawmakers to release $350 billion for the Treasury Department's financial rescue. His team expects the economic-recovery package to pass within a month, and has promised more rigorous oversight of the bailout funds. The Senate, meanwhile, is expected to confirm Obama's major cabinet nominees as soon as Tuesday.
Sturdy approval ratings could give the new president an open window to cross the partisan divide in Congress and push through the dramatic reforms he says are necessary. Obama's popularity is in stark contrast to the man he is replacing. George W. Bush is the least-popular outgoing executive since Richard Nixon, his standing with the public hammered by the recession, war and a banking collapse that critics link to his administration's embrace of lenient financial regulations.
Obama will need all the goodwill he can muster.
Ken Sherrill, a political science professor at Hunter College in New York, said Obama's problems are even more complex than Roosevelt's.
"There wasn't any war when FDR took office," Sherrill said. "This requires an ability to multitask that we haven't seen before."
Despite the degree of difficulty, Americans' expectations for Obama are high, in part because they believe the government will step into areas where it has historically been more reluctant to venture - from home foreclosures to ailing industries.
Though Obama enjoys healthy approval ratings at the start of his presidency, he will still face robust opposition, from Republicans leery of his stimulus package and Democrats worried about budget deficits. On Inauguration Day, one conservative group began running a national television advertisement critical of increased government intervention. Its title: "This Was Not the Change We Were Promised."
But there was little sign of opposition Tuesday in Washington. Millions thronged to the Capitol for the inauguration, enduring gridlocked roads and jampacked subway cars for a chance to glimpse the new president. Most were content to watch the festivities on jumbotrons spread around the National Mall, soaking in the festive atmosphere with fellow travelers.
On the massive podium to witness Obama's historic moment were a group of A-list musicians, actors and athletes, ranging from Jay Z to Puff Daddy, Smokey Robinson, Beyonce and basketball great Magic Johnson.
Actor Denzel Washington, an early arrival, summed up the mood: "I'm happy, I'm cold and I'm celebrating," Washington said.
The heavy weight of history of the event, which comes a day after the celebration of Martin Luther King Jr.'s birthday, was marked by a notice by the Homeland Security Department of a "potential threat" of uncertain credibility.
The new president and Vice President Joe Biden began the day with services at St. John's Episcopal Church, just across Lafayette Square from the White House. The men and their wives, Michelle Obama and Jill Biden, then joined Bush and Vice President Dick Cheney at the White House for coffee, before heading to the Capitol for the swearing-in ceremony.
Following the swearing-in, Obama will have lunch in the Capitol, review the troops at the East Front of the Capitol, then watch the inaugural parade. The Obamas will attend all 10 official inaugural balls later Tuesday.
Obama spent the days ahead of the inauguration with a blitz of activities - both festive and solemn. On Saturday, the president-elect took a whistle-stop train ride from Philadelphia to Washington, stopping for jampacked rallies in Wilmington, Del., and Baltimore. A day later, he laid a wreath at Arlington National Cemetery and took in a star-studded concert at the Lincoln Memorial. On Monday, Obama visited wounded troops and volunteered at a shelter for homeless teens.