Millions of people are gathering in the US capital to see Barack Obama sworn in as America’s 44th president – and its first African-American leader.
As dawn broke in a cold Washington DC, crowds were packing into the National Mall to witness Mr Obama take the oath of office at exactly 1200 (1700 GMT).
Unprecedented security is in place, with 40,000 security personnel either on duty or stand-by in the city.
Across the US, there is a sense of history being made, correspondents say.
The roar of approval that will greet Barack Obama when he takes the oath of office will be genuine, heartfelt and will come from all corners of America, BBC North America editor Justin Webb says.
But the new president faces serious challenges. America is gripped by uncertainty as the economy faces its worst crisis in decades. Mr Obama must also handle wars in Iraq and Afghanistan, and the Israeli-Palestinian conflict.
Anticipation has been building across the US ahead of the ceremony, held on the steps of the US Capitol, where Congress sits.
Hundreds of thousands of visitors have been arriving in Washington in recent days. Competition for tickets along the parade route has been fierce.
Inauguration Day began early for Mr Obama and his wife, Michelle. Before 0900 local time (1400 GMT), the couple travelled in a heavily-armoured motorcade for a private church service at St John’s Episcopal Church alongside Vice President-elect Joe Biden and his family.
The service will be followed by coffee at the White House with outgoing President George W Bush and Vice-President Dick Cheney.
The group will then travel to the US Capitol along a packed route. There Mr Biden will be sworn in, followed by Mr Obama.
America’s first black president will place his hand on a Bible used by Abraham Lincoln at his inauguration in 1861 and repeat the oath of office, promising to “preserve, protect and defend the Constitution of the United States”.
He will then deliver his inaugural address – a 20-minute speech which, aides say, will focus on dual themes of responsibility and accountability.
US BLACK HISTORY MILESTONES
1861 – Civil war starts
1865 – Civil war ends, leading to abolition of slavery
1870 – Hiram Revels is first African-American senator. African-American men gain the vote, but face resistance and intimidation
1954 – Civil disobedience campaign starts
1963 – Martin Luther King: “I have a dream” speech
1964 – Civil Rights Act
1965 – Right to vote guaranteed
Crowds in the National Mall will watch proceedings on huge video screens. At least two million people are expected, a record number for an inauguration event.
They will be braving unusually cold weather, with temperatures of -1C expected. Wind chill would make it feel several degrees colder, forecasters said, urging people to guard against possible hypothermia and frostbite.
Early on Tuesday, tens of thousands defied the pre-dawn cold to secure a good spot in the Mall when it opened at 0400 (0900 GMT) to those without tickets.
There was a huge sense of excitement, a BBC reporter at the scene said, with people breaking out in cheers. The subway was as busy at 0500 as it would normally be at rush hour, but most seemed happy to battle through the crowds.
“I’ve been queuing for hours, I don’t care how long I have to wait,” Washington resident Ronald Brisbon, 55, told the BBC.
“Dr [Martin Luther] King said it might take 40 years. It’s been 45 years, I can wait another hour.”
Security chiefs say they are prepared for all eventualities. Roads and bridges into Washington will be closed and thousands of police, soldiers and plainclothes agents are on the streets.
OBAMA’S ECONOMIC CHALLENGE
Unemployment rate up to 7.2% – 16-year high
Retail sales fell for six months in a row in December – down 2.7%
Car sales down to 22.4% below level seen a year ago
New home sales in November at lowest level in 17 years
Mid-price of a new home sold in November: $220,400 (Ã‚Â£149,900) – down 11.5% from a year ago
Trade deficit dropped to $40.4bn (Ã‚Â£28.82bn) in November – five-year low
Snipers will be in position along the parade route, while helicopters and fighter jets will patrol the skies.
The feeling among the crowds in Washington, correspondents say, is that the changing of the presidential guard will be far more than the sum of its ceremonial parts.
As if to emphasise the historic significance of the occasion, the eve of Mr Obama’s inauguration coincided with the annual public holiday in memory of black civil rights leader Martin Luther King.
As he helped decorate a community project in Washington in memory of Dr King, who was assassinated in 1968, Mr Obama called on Americans to work together amid the economic downturn.
“Given the crisis that we’re in and the hardships that so many people are going through, we can’t allow any idle hands,” he said.