Barack Obama sought "a new way forward with the Muslim world" on Tuesday in an inaugural speech that coincided with stark scenes of devastation in the aftermath of the 22-day war in Gaza.
Stepping into history as the first black president of the United States, Obama began his inaugural address by thanking predecessor George W Bush for his "service to the nation."
Appealing for "hope over fear, unity of purpose over conflict and discord", Obama said every man, woman and child who seeks peace is a friend of America
"We seek a new way forward with the Muslim world based on respect," said the 44th US president who took oath pronouncing his full name 'Barack Hussain Obama'
But, he stressed: "We remain the most powerful nation on earth."
To those who seek to harm and destroy America, Obama said "We'll defeat you."
A smiling Obama placed his hand on a Bible shortly after noon (EST), and promised to "preserve, protect and defend" the US constitution, as America's 44th president and the country's first African-American leader.
Hundreds of thousands, bundled up against the cold, packed Washington's Mall, which stretches 2 miles (3 km) from the Capitol to the Lincoln Memorial on the Potomac River and along Pennsylvania Avenue to the White House.
Television pictures showed a sea of people, many waving American flags, in the grassy plain of the Mall as Obama was sworn in taking over from Republican George W. Bush, riding a wave of public optimism he will need to tap to deal with the worst economic crisis in 70 years and two wars.
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"It is amazing to be here. This is history in the making," said Judy Bailey, 42, of Cincinnati, Ohio.
The inauguration of Obama, 47, the son of a black Kenyan father and a white mother from Kansas, will be full of ceremony, and full of symbolic meaning for African-Americans, who for generations suffered slavery and then racial segregation that made them second-class citizens.
Obama took his oath of office with his hand on a Bible used by Abraham Lincoln at his first inauguration in 1861, standing on the western steps of the capitol, a building that was partly built by black slaves.
His swearing-in was followed by the U.S. Marine Corps band playing "Hail to the Chief" and a 21-gun salute.
Some estimates put the number of people packing the Mall and inaugural parade route at more than 2 million. Crowds clogged the city's metro rail system and thronged the security check-points.
Polls show widespread support for Obama and optimism about the coming four years of his presidency, despite a deepening recession that has saddled the country with a $1 trillion deficit and 11 million people unemployed.
Bush, who is also leaving a legacy of wars in Iraq and Afghanistan, is departing office after two terms having plumbed record lows in his approval ratings.
GETTING DOWN TO WORK
Obama, a former Illinois senator who capped a hard-fought election campaign with a comfortable win against Republican John McCain in November, has stressed that the time to party and celebrate will be short-lived.
On Wednesday he will start shepherding an $825 billion economic stimulus plan through Congress.
He faces daunting challenges -- global economic turmoil, the wars in Iraq and Afghanistan, global warming, conflict in Gaza and tensions between Pakistan and India.
But analysts say that at least for now, it will be his ability to revive the recession-hit economy that will be the benchmark by which his presidency is judged.
The Washington Post reported that in one of his first actions as president, Obama planned to name former Sen. George Mitchell as his Middle East envoy in one of his first actions as the new U.S. president.
Obama is expected to meet his military commanders on Wednesday to discuss the U.S. troop withdrawal from Iraq, to fulfill a campaign pledge to withdraw all troops within 16 months.
"He is going to lay out a lot of the challenges we face as a country, but he will also remind ... that America has faced great challenges before," his spokesman Robert Gibbs told CNN on Tuesday.