Barack Obama will pay homage to slain civil rights leader Martin Luther King today as he vows to revive the spirit of sacrifice to overcome war and economic crisis during his upcoming presidency.

As Americans celebrate Martin Luther King, David Plouffe, manager of "Obama for America," said the president-elect planned to spend the day highlighting community service.

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"Help kick off an ongoing commitment to serve our communities by taking part in this extraordinary day of service," Plouffe said in an appeal to Obama followers. "Sign up to attend or host an event in your community and help rebuild America one neighborhood at a time."

A new CNN/Opinion Research Corp. poll showed that nearly seven in 10 African Americans believe that with the election of Obama, Martin Luther King's dream of racial equality has been fulfilled. Meanwhile, 84 per cent of those surveyed said they approved of how Obama is handling the presidential transition.

Yesterday, the president-elect stood in the shadow of the Lincoln Memorial, dedicated to the slain president who brought the United States intact through the Civil War and abolished slavery, and gave a somber assessment of the perils ahead despite the exuberant mood among the crowd.

"In the course of our history, only a handful of generations have been asked to confront challenges as serious as the ones we face right now. Our nation is at war. Our economy is in crisis," he said.

"I won't pretend that meeting any one of these challenges will be easy. It will take more than a month or a year, and it will likely take many," Obama said.

"But never forget that the true character of our nation is revealed not during times of comfort and ease, but by the right we do when the moment is hard.

"I ask you to help reveal that character once more, and together, we can carry forward as one nation, and one people, the legacy of our forefathers that we celebrate today."

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U2, Bruce Springsteen and Stevie Wonder -- whose songs were totemic anthems of Obama's barnstorming rise to power -- headlined a concert for a sea of people standing in arctic cold in front of Abraham Lincoln's memorial.

It was part of an unprecedented run-up to Obama's inauguration that has generated an outpouring of public enthusiasm and high hopes even as the country faces two wars and a dire economic crisis.

The hundreds of thousands attending were the advance guard of an inaugural crowd expected to number millions, as an elaborate security operation began with police and army reservists taking up position across the US capital.

In contrast to the jubilant afternoon party, Obama struck a more somber note as he joined vice president-elect Joseph Biden in laying a wreath at the Tomb of the Unknown Soldier in Arlington Cemetery.

Dressed in black winter coats to guard against the freezing temperatures, Obama and Biden held their hands over their hearts as "Taps," the US military's haunting lament to the fallen, was played by a lone bugler.

Obama and his wife Michelle then climbed into the incoming president's new armored Cadillac with a blue license plate reading "44" -- his numerical position as the newest commander-in-chief.

Along a railroad from Philadelphia to Washington, a route once traced by his hero Lincoln, Obama on Saturday urged Americans to adopt a new "Declaration of Independence" from bigotry, small thinking and ideology.

Aides said those themes will figure large in Obama's inaugural address after he is sworn in around noon tomorrow.

Rahm Emanuel, the next White House chief of staff, told NBC the speech would declare an end to "the culture of anything goes" and demand a new era of responsibility from government, corporate boardrooms and the American people.

Incoming White House press secretary Robert Gibbs vied to temper sky-high expectations both worldwide and at home, as the United States grapples with its longest recession since World War II.

"We did not get into the situation overnight. The problems and the challenges that our country face didn't happen all last week. It's going to take us some time," Gibbs said on "Fox News Sunday".

But Gibbs and other senior aides said Obama would act rapidly to enact his economic revival plans via a mammoth stimulus package worth 825 billion dollars.

And on his first full day in office Wednesday, the aides said, Obama will also convene his top military brass to map a way out of Iraq and recommit US troops against a resurgent Taliban in Afghanistan.

After the tumultuous Bush years, Obama comes to office with the highest poll ratings since Ronald Reagan in 1981. The New York Times and CBS News said 79 percent of respondents to its poll were optimistic about the next four years.