After the swearing-in ceremony and inaugural address, President Obama stepped out of his limousine to greet part of the enthusiastic crowd that lined the route of his inaugural parade.
Obama and his wife, Michelle, got out of their car on Pennsylvania Ave., waved to the cheering spectators on both sides of the street, and then walked part of the way, smiling and waving.
A couple of moments later, Vice President Joe Biden and his wife, Jill, joined them on the walk.
President Jimmy Carter was the only president to march all the way along his inaugural parade route, reports CBS Evening News anchor Katie Couric, though Bill Clinton and George H.W. Bush did walk part of the way.
Along the way, hundreds of people who had been packed on rooftops and balconies broke into loud cheers. They got an unobstructed view of the parade from above the large crowd.
The four walked for about seven minutes before getting back into their limousines.
The parade route has also been lined with police and military personnel, who have saluted as the new president's limousine slowly moved past them.
At first light hundreds of thousands of people were already streaming onto the National Mall, reports CBS News Capitol Hill correspondent Chip Reid.
While the President-elect and First Lady-to-be had coffee with the outgoing President and his wife, the flood of humanity continued, despite weather so cold the Reflecting Pool at the Washington Monument was frozen solid. The crowd eventually reached about 2 million, five times the number that turned out four years ago for the second inauguration of George W. Bush.
By comparison, in 1981, President Ronald Reagan's inauguration drew about 500,000 people, and President Bill Clinton's 1993 inauguration drew about 800,000 people, according to National Park Service estimates.
On the National Mall, the crowd stretched nearly two miles - from the Capitol building to the steps of the Lincoln Memorial, where civil rights leader Martin Luther King Jr. delivered his "I Have a Dream" speech.
The bulk of the crowd was jammed into the area between the west front of the Capitol and the Washington Monument, where people stood shoulder-to-shoulder as Barack Obama was sworn in as the 44th president. The crowd was so tightly packed that some people complained they felt claustrophobic.
The vast, excited crowd bore witness to a transfer of American power like none before it. The blare of regal trumpets and thunder of cannon were familiar. The transition from Republican to Democrat, and gray hair to dark, had happened before.
But this was white to black, a shattering of racial barriers finally made complete when Barack Obama made it through a bumbled oath-taking, delivered a momentous-by-definition speech and got back to being his unflappable self.
The Democrat who charged onto the national scene saying this was not a nation of red states and blue states, but the United States, became president while wearing a red tie, the Republican color.
Republican George W. Bush, president no more, wore a blue tie, the Democratic color. They embraced at the Capitol and walked out together.
The racial milestone lent a deeply personal dimension for many in the crowd as well as a historical landmark for all.
New York Governor David Patterson told Couric that Obama's inauguration was "a fulfillment of a dream that African-Americans, both the living and the dead, have struggled for, for a couple of centuries to build a movement that would bring economic, political and social justice to African-Americans.
"But really what touched me was the effect that this inauguration has had on white Americans. People who didn't vote for President Obama, presumably might not vote for him for re-election and wouldn't agree with his policies still see the historical significance."
"I've been real emotional all morning thinking about my grandmother and the heroes whose shoulders we stand on," said Lyshundria Houston, 34, here from Memphis, Tenn., after more than 20 hours of travel. Houston, who is black, said: "They'd be so proud."
Coming from the city where Martin Luther King Jr. was assassinated, she reflected on the civil rights movement on her way to the parade, and said: "Sometimes that makes the cold go away."