Washington  - Barack Obama expects to enlist the help of ordinary Americans after he takes office Tuesday as the country's first African-American president and inherits the country's worst economic crisis in decades.

That message was hammered home Monday as Obama rolled up his sleeves and joined other Americans in celebrating the birthday of civil rights leader Martin Luther King Jr with a nationwide call to community service.

The outpouring of generosity comes amid final preparations in Washington for the lawmakers, dignitaries and as many as 2 million Obama supporters who were expected to flood the capital's National Mall for the swearing-in ceremony on Tuesday.

Obama, 47, has sought to play down the sky-high expectations of his presidency, warning that the economy will likely "get worse before it gets better" and urging all US citizens to do their part to turn the country's fortunes around.

"We're gonna' have to take responsibility, all of us," Obama said in a stop at Calvin Coolidge High School in Washington, which was writing letters and decorating gifts for US soldiers serving abroad. "If we're just waiting around for somebody else to do it for us ... it never gets done."

Obama turned this year's celebration of King's legacy into a "day of service" - a prominent theme during his two-year campaign for the presidency and an attitude he promised to instill throughout his time in the White House.

Obama said the day's theme was based on King's own life, which was "lived in loving service to others." For African Americans, Obama's inauguration will go a long way towards fulfilling the hopes of racial equality that King evoked in his famous "I Have A Dream" speech in 1963.

Obama began the day visiting wounded US soldiers at Walter Reed Medical Center and later helped paint the walls of Sasha Bruce Youthwork, a homeless shelter in Washington for teenagers.

"Given the crisis that we are in and the hardships that so many people are going through, we cannot allow any idle hands," Obama said during the stop. "Everybody is gonna' have to pitch in."

Obama's wife Michelle as well as vice president-elect Joe Biden and his wife Jill were out in Washington to attend other service projects.

More than 5,000 volunteer groups had helped organize 11,000 similar projects, involving as many as one million people across the country, Obama said.

"Finally, we have someone who really does care about the people, and it makes you want to serve," said Sanaa Msemagi, a native Californian who was collecting canned goods outside the Martin Luther King Jr library in Washington.

In the evening, three high-profile Americans were to be honoured for their own service to the country: Colin Powell, a Republican and former US secretary of state; John McCain, Obama's Republican rival for the presidency and a Vietnam War hero; and Biden, a 36-year veteran of the US Senate.

Millions from across the country have descended on Washington to witness Obama's historic inauguration as the 44th president, filling hotel rooms as far as 400 kilometres away from the city.

Huge numbers of people roamed the mall already Monday.

Long lines of lucky ticket recipients for the inauguration waited outside congressional offices on Capitol Hill to get their passes, and crowded into nearby subway entrances for 20-minute waits just to get onto the platform.

The inauguration party already got underway Sunday when hundreds of thousands of people flocked to the Lincoln Memorial for a two-hour concert, the first of a series of festivities leading up to the presidential oath of office on the Capitol steps.

The star-studded lineup for the free, public show on a freezing afternoon under cloudy skies featured performances by Beyonce, Bono, Bruce Springsteen and Stevie Wonder, but the real star of the day was no singer.

Obama watched the concert with his wife Michelle and two children and received a huge ovation when he arose, waved to the crowd and strode to the podium to address the masses.

The concert was a sort of warm-up event for authorities, who are preparing for even larger crowds and even colder weather on the Mall on Tuesday. The festivities area opens for the public at 4 am Tuesday morning.

President George W Bush has no public events planned on his last day in the White House. As per tradition, Bush will accompany Obama to his swearing-in Tuesday before making a low-key departure to his Texas home. (dpa)