Hundreds of thousands of people have streamed into Washington to witness the inauguration of Barack Obama as the first black US president, in a moment of national unity after years of political tumult.
Vast crowds braving freezing cold crammed the National Mall and mobbed streets leading onto the central thoroughfare, clogging the Washington metro system and roads leading to downtown more than five hours before Obama was to take the oath of office on Tuesday.
In a sign of the magnetism sparked by Obama's ascendancy, millions were expected to see Obama, the son of a black Kenyan father and a white American mother, swear to "preserve, protect and defend" the US constitution.
The US Capitol, in front of which Obama will take the oath, bearing the hopes of a demoralised nation, was lit up in brilliant white lights, as streaks of red and blue heralded the dawn.
The former Illinois senator was set to step up to a dais in front of the building just before noon (0400 AEDT Wednesday) to assume power from President George W Bush after two terms marked by stark political division.
He will inherit an economy crippled by decline, two foreign wars and massive challenges to US authority overseas.
On Monday, Obama paid tribute to Martin Luther King, on the holiday honouring the civil rights icon's birth.
"We will come together as one people on the same mall where Dr King's dream echoes still," Obama said.
"As we do, we recognise that here in America, our destinies are inextricably linked," he said.
"We resolve that as we walk, we must walk together. And as we go forward in the work of renewing the promise of this nation, let's remember King's lesson - that our separate dreams are really one."
In the first presidential handover since the September 11 attacks of 2001, Obama was set to be sworn in by Chief Justice John Roberts.
Then, the new president was to deliver his most important speech yet in a career sprinkled with memorable oratory since his explosion onto the national stage in 2004.
Aides said Obama's call for all Americans to embrace public service would dominate his inaugural address, as he gets to grips with the nation's deepest recession since World War II and his plans to pull US troops out of Iraq.
A BBC poll of people in 17 countries found an average of two-thirds believe Obama will improve frayed relationships between the United States and the rest of the world.
But with expectations running high at home and globally, Obama's team is pleading for patience as it confronts a groaning in-tray of challenges from Gaza to Guantanamo.
The celebrations had an acute poignancy for many in the United States, and the world, given Obama's mould-shattering biracial heritage. But there was also a dose of realism of what the new administration can expect.
"I am deeply convinced that the biggest disappointments are born out of big expectations," Russian Prime Minister Vladimir Putin said.
Following the inauguration of Obama, 47, and vice president-elect Joseph Biden, 66, the new US leaders were to lunch with members of Congress, Supreme Court justices and Obama's cabinet, including secretary of state Hillary Clinton.
Marching bands, military veterans, union workers and schoolchildren were to then join a parade down Pennsylvania Avenue to the White House for Obama to take up the reins of power in the Oval Office and his place in history.
Outgoing Vice President Dick Cheney, 67, pulled a muscle in his back on Monday - the latest in a series of health problems - while moving boxes and will be in a wheelchair for the inauguration, the White House said.
The whirlwind day was to end with 10 official inaugural balls before the Obamas could retire with their daughters Malia and Sasha, becoming the youngest first family since John F Kennedy's in the early 1960s.
The crowds of festive inaugural watchers had to navigate jammed routes on the approaches to Washington, blocked-off roads downtown and crammed metro trains. Armoured personnel carriers were a visible sign of smothering security.
"We woke up at three but it's worth it," said Mary Lloyd. "I think it's history on its way. We just want to be there celebrating."
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