Washington  - Tears flowed and the crowd roared as Barack Obama took the presidential oath on a subfreezing Tuesday and signalled the beginning of a new era in Washington under the Democratic president and the nation's first African American to live in the White House.

The boisterous crowd, which had booed when outgoing Republican US President George W Bush stepped onto stage, fell briefly silent at the moment Obama swore the oath.

Then it exploded with loud cheers and yells across the packed National Mall, the 3-kilometre stretch that ties Congress to the Lincoln Memorial.

Three African-American women collapsed on the ground in tears and emotion, while the men in their group stood by and openly wept.

Obama, 47, and Vice President Joe Biden were sworn in by justices of the Supreme Court as Bush watched from the sidelines. Bush, one of the most unpopular presidents in recent times, boarded a helicopter behind Capitol Hill after the ceremony as Obama and wife Michelle waved goodbye.

Vice President Dick Cheney, possibly one of the most controversial members of Bush's administration for his urging and defending the war in Iraq, appeared at the ceremony in a wheel chair and wielding a cane - an image in which many critics may have found a sense of poetic justice.

He had injured himself moving a carton out of the vice presidential residence.

Senator Diane Feinstein, who chaired the congressional inauguration organization, opened the ceremony on the steps of the domed Capitol building that was built by 18th- and 19th-century black slaves.

She noted the US tradition of a peaceful transition of power, symbolized earlier by the Obama's visit to the White House for coffee with the Bushes before the outgoing and incoming presidents rode together in a limousine to Capitol Hill.

Further sealing the "peaceful transition" theme, former presidents Jimmy Carter, George HW Bush - the father of the outgoing president - and Bill Clinton were also in the crowd. Clinton's wife, Hillary, is Obama's former rival for the Democratic nomination and his choice for secretary of state in his new administration.

"We gather to etch another line in the solid stone of history," Feinstein said. "Future generations will mark this morning as the turning point for real and necessary change in this nation."

She noted also the special historic nature of Obama's presidency as an African American, calling it a "dream that echoed across history (and) finally reached the walls of the White House."

Obama took his oath of office on a Bible used by president Abe Lincoln, the Civil War president who freed the slaves.

Afterwards, the reaction was more muted in the VIP ticketed area than among the millions in the public areas on the Mall. But celebrities, who included musicians Beyonce, P Diddy and JayZ and actors Samuel L Jackson and Denzel Washington, hugged each other after the oath.

Actor Washington sought isolation at the moment of the oath, standing up in the middle of a corridor to gaze toward Obama. Others wept and blew their noses.

Before the oath, as they ambled in to their special seats, members of Congress and other officials took out their personal cameras and took photos of the human sea that flowed from the Capitol to the Lincoln Memorial.

There was even applause for the prayer of the controversial Reverend Rick Warren, whose outspoken opposition to gay rights provoked anger among activists. Warren said that the country was celebrating "a hinge of history with the inauguration of our first African American president."

"We are so grateful to live in this land, a land of unequal possibility where the son of an African immigrant can rise to the highest level of our leadership," Warren prayed.

One man, Darwin Johnson, 48, a maitre de from San Diego, California, was among the lesser known of the 240,000 guests in the ticketed area. The tickets were distributed by Congress.

"We changed the direction of this country and by proxy the direction of the world," Johnson, an African American, said. "I feel like a kid in Disney World. And I don't want to go back to school."

Obama has vowed to restore America's faded moral standing in the world after eight years under Bush. That idea was picked up by Congressman Jesse Jackson Junior, whose civil rights activist father ran for president in the 1980s.

"Today, the world will look at America differently and today Americans will look at each other differently," he told Deutsche Presse-Agentur dpa (dpa)