WASHINGTON (AP) — 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 noon Tuesday 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. Tuesday to take her place on the Mall for the swearing-in.

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 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 arranged to attend dinners for Vice President-elect Joseph Biden and former Secretary of State Colin Powell, a Republican.

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 Tuesday's ceremonies.

George W. Bush was in the White House for a final full day as president after two terms marked by the terror attacks of Sept. 11, 2001, 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.

On Tuesday morning, he and first lady Laura Bush will greet Obama and his wife, Michelle, at the storied 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.

There was one unscripted moment Monday, when Jill Biden blurted out while taping the Oprah Winfrey show that her husband had been offered a choice of vice president or secretary of state by Obama.

"Shhh!" said the vice president-elect, whose attempt to silence his wife sent the audience into laughter. A few hours later, his office issued a statement that said: "To be clear, President-elect Obama offered Vice President-elect Biden one job only — to be his running mate. And the vice president-elect was thrilled to accept the offer."

For the most part, the day 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."

Obama's public events recalled his time as a community service organizer in Chicago, in the years before he entered politics. An Illinois state senator a mere four years ago, he won election to the U.S. Senate in 2004 and announced his candidacy for the White House in early 2007.

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. Several blocks from where Obama will take the oath of office, the Boy's Choir of Kenya gave an impromptu performance for anyone who happened to be nearby.

"Mission Accomplished," read the headline on a souvenir magazine that bore the picture of Obama and his wife.

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. "The cadence and syntax of Obama, it comes directly from Dr. King," said Kirk Moses, a retired high school teacher as others in his group took pictures of the bronze plaque marking the spot for the future memorial.

Bush spokeswoman Dana Perino said Defense Secretary Robert Gates had been designated to stay away from Tuesday's inaugural festivities "in order to ensure continuity of government." By custom, one government official stays away when others in the line of presidential succession attend public events. Gates has been tapped to remain in his post by Obama.