WASHINGTON -- Barack Obama stood at the threshold of the White House on Monday, summoning fellow Americans to join him in service as tens of thousands flocked to the nation's capital to celebrate his inauguration as the first black president.

"Tomorrow we will come together as one people on the same Mall where Dr. King's dream echoes still," said the president-elect, invoking the memory of the Rev. Martin Luther King Jr. on the national holiday in his honor.

The 47-year-old former senator takes office at 11 a.m. today at a time of economic crisis at home and two wars overseas. But the challenges of the moment receded into the background for many who came for a chance to witness history.

"I didn't think I would see a black president in my generation. I just had to be here," said Donald Butler, 20, a University of Washington student.

"I'm just really happy that I'm living to see this wonderful event," said 70-year-old Betty Bryant, who rode a chartered bus from Augusta, Ga. Standing in front of the icy Reflecting Pool facing the Capitol, she made plans to rise at 2 a.m. today to take her place on the Mall for the swearing-in.

Ron Edmonds ~ Associated PressPeople visit the National Mall near the Washington Monument as preparations are underway Monday for president-elect Barack Obama's inauguration in Washington, D.C.

Obama's own schedule showed he had one eye on the bipartisan support he'll need after taking office. The night before becoming the nation's 44th president he was stopping by dinners saluting his vice president, Joe Biden, and two Republican stalwarts: former secretary of state Colin Powell and Sen. John McCain of Arizona, the man Obama defeated last November to win the White House.

Uniformed military personnel patrolled Washington street corners, the advance guard of a massive security presence planned for the oath-taking, inaugural speech, parade and other festivities. Officers checked out some suspicious packages and vehicles, but everything was cleared, said FBI spokesman Richard Kolko.

"The city and the people seem to be in a good mood and good spirits," he said. "Security is going well, that's what all the planning is for."

On the specially built inaugural stands outside the Capitol, musicians Yo-Yo Ma and Itzhak Perlman rehearsed for their role in today's ceremonies.

President Bush was in the White House for a final full day as president after two terms marked by the terror attacks of Sept. 11 and the economic collapse of 2008. He placed calls to world leaders, including Russian Prime Minister Vladimir Putin, and commuted the prison sentences of two former Border Patrol Guards.

Today, he and first lady Laura Bush will greet Obama and his wife, Michelle, at the mansion's front portico and see them in for a brief visit. By the time Obama returns at mid-afternoon, he will be the nation's 44th president, Bush will be en route to a Texas retirement and the moving vans will have departed with one family's belongings and arrived with the other's.

For the most part, Monday went according to a script designed to emphasize the theme of community service.

Obama issued a statement declaring, "King's was a life lived in loving service to others. As we honor that legacy, it's not a day just to pause and reflect -- it's a day to act."

He began his day with a visit to Walter Reed Army Medical Center, where he visited 14 veterans injured in Iraq and Afghanistan.

Then his motorcade headed for the Sasha Bruce House, a facility for homeless teens, where he grabbed a paint roller and helped volunteers who were fixing up rooms.

"We can't allow any idle hands," he said. "Everybody's got to be involved."

Later, Obama joined his wife at a high school where they greeted 300 volunteers who were writing letters to U.S. troops and doing other volunteer work.

The president-elect thanked them and said, "If we're waiting for somebody else to do something, it never gets done. We're going to have to take responsibility, all of us. This is not just a one-day affair."

Referring to his imminent new job, he said, "I am making a commitment to you as your next president that we are going to make government work."

A party atmosphere took hold on the National Mall. As light snow fell intermittently, small crowds gathered around large-screen televisions showing reruns of Sunday afternoon's concert featuring a star-studded cast.

Two wreaths materialized at the site of a future memorial to King on the Mall in an area on the Tidal Basin between the Jefferson Memorial and Lincoln Memorial.

Kirk Moses, a retired high school teacher visiting the spot of the future memorial, said, "The cadence and syntax of Obama, it comes directly from Dr. King."