enlarge The Associated Press

President-elect Obama takes the stage today before introducing Sen. John McCain at a dinner honoring the Arizona Republican in Washington.

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.

"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 Illinois senator is taking office 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 3 a.m. today to take her place on the National Mall for the swearing-in ceremony.

For Obama, the day was stripped of partisan politics, and he ended it by lavishing praise on Republican Sen. John McCain of Arizona, his opponent in last fall's presidential campaign. In remarks prepared for a dinner in honor of his one-time rival, he called the former Vietnam prisoner of war a "rare and courageous public servant" who places country before party.

"Let us strive always to find that common ground, and to defend together those common ideals, for it is the only way we can meet the very big and very serious challenges that we face right now," said Obama, who also attended dinners for Vice President-elect Joseph Biden and former Secretary of State Colin Powell.

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.

George W. Bush was in the White House for a final full day as president. He placed calls to world leaders, including Russian Prime Minister Vladimir Putin.

This morning, 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 midafternoon, 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.

The day went according to a script designed to emphasize a theme of community service.

Obama issued a statement: "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....