By Matt Spetalnick and Caren Bohan
WASHINGTON (Reuters) - On the eve of his historic inauguration, Barack Obama joined on Monday in honouring slain civil rights leader Martin Luther King, underscoring racial barriers the Illinois Democrat overcame to be elected the first black U.S. president.
Taking time away from preparing for an address he will deliver when he is sworn in on Tuesday, Obama visited wounded troops at a military hospital and issued a call to Americans to remember King by recommitting themselves to public service.
Hundreds of thousands of visitors streamed into Washington for inaugural festivities but the celebration was tempered by the daunting challenges Obama will face -- unfinished wars in Iraq and Afghanistan and the worst financial crisis since the Great Depression.
Obama's inauguration, coming back-to-back with Monday's federal holiday honouring King, has added to the deep symbolism of an African-American receiving the keys to the White House, which was built partly with the labour of black slaves.
"Today, we celebrate the life of a preacher who, more than 45 years ago, stood on our national mall in the shadow of Lincoln and shared his dream for our nation," Obama said in a statement.
"Tomorrow, we will come together as one people on the same mall where Dr. King's dream echoes still. As we do, we recognise that here in America, our destinies are inextricably linked. We resolve that as we walk, we must walk together."
In crafting one of the most eagerly awaited inaugural addresses ever, Obama will try to reassure recession-weary Americans they can rebound from hard times, and he will signal to the world his desire to fix a battered U.S. image overseas.
But Obama, elected on a promise of change after eight years under Republican President George W. Bush, will also be mindful that if he sets expectations too high, he could risk disappointment.
Obama has vowed to spend hundreds of billions of dollars to jolt the economy out of the doldrums, and has said he wants to bring U.S. combat forces out of Iraq within 16 months.
After rolling up his sleeves to help paint a wall at a shelter for homeless and runaway teenagers, Obama touched on a theme of personal responsibility expected to figure prominently in Tuesday's speech from the U.S. Capitol steps.
"Given the crisis that we're in and the hardships that so many people are going through, we can't allow any idle hands. Everybody's got to be involved," he told reporters. Obama was accompanied by King's son, Martin Luther King III.
Obama's relatively smooth transition to power suffered a hiccup when Texas Republican Sen. John Cornyn said he may hold up Hillary Clinton's nomination to become secretary of state if his concerns about foreign donations to former President Bill Clinton's foundation are not resolved.
MILESTONE IN U.S. RACIAL HISTORY
The inauguration of Obama, son of a black father from Kenya and a white mother from Kansas, will mark a milestone in America's turbulent history of race relations.
It will come more than four decades after the height of the civil rights movement led by King, who preached racial harmony and was assassinated in 1968 by a white supremacist.
Obama often invoked King's legacy during his rise from unlikely candidate to his election in November as the country's 44th president.
But Obama also sought during much of the campaign to avoid calling direct attention to race, which remains divisive in U.S. society, and he came to be known to some as a "post-racial" politician.
A record crowd is expected for his inauguration, with a million people likely to fill the National Mall, a vast green surrounded by museums and monuments, and thousands more lined up along a parade route to the White House.
An unprecedented security operation was already under way, including patrols on ground, air and water.
Parties, concerts and seminars marking Obama's inauguration were launched over the weekend and will hit full stride after Tuesday's ceremony, which will be covered by U.S. broadcasters as Americans' version of a royal coronation.
At the White House, Bush administration staffers were packing up while the president kept a low profile.
An activist coalition calling itself "Arrest Bush" piled a motley collection of dozens of old shoes -- including tan combat boots said to have been worn by U.S. troops in Iraq and children's yellow flip-flops -- outside the White House fence.
"We wanted to shoo and boo Bush on his last day in office," said Ann Wilcox of Washington, D.C., who marched with the group of about 500 peace activists.
Bush leaves office with some of the lowest approval ratings of any modern president and with some historians already saying his tenure will rank among the worst ever.
Unlike some predecessors, Bush was tight-fisted with last-minute presidential pardons. He opted only to commute the sentences of two Border Patrol agents convicted for shooting an unarmed Mexican drug smuggler in the buttocks. The case drew an outcry from supporters who said the men were doing their jobs.
Bush's final official act will be to welcome Obama to the White House before the swearing-in and accompany him there by motorcade to attend the ceremony before flying home to Texas.
(Additional reporting by Tabassum Zakaria and Jim Wolf, editing by David Wiessler)