Barack Obama, swearing an oath to preserve and protect the Constitution, became the nation's 44th president at noon today in a solemn ceremony almost as old as the nation itself.

At that moment a roar rose up from more than 1 million spectators who had jammed on the mall for two miles west of the capitol. The U.S. Marine Band struck up Hail to the Chief and cannons boomed a 21-gun salute to the first African American president.

Obama, laying his hand on the Bible that President Abraham Lincoln used in his 1861 inauguration, was administered the oath by Chief Justice John Roberts.

In his 18-minute inaugural address, Obama called for a new era of responsibility and community, saying the nation must choose "hope over fear, unity of purpose over conflict and discord."

"Our time of standing pat, of protecting narrow interests and putting off unpleasant decisions - that time has surely passed," Obama said. "Starting today, we must pick ourselves up, dust ourselves off and begin the work of remaking America."

Repeating his call for change, Obama said, "We seek a new way forward, based on mutual interest and mutual respect."

He also warned, "To those leaders around the globe who seek to sow conflict, or blame their society's ills on the West - know that your people will judge you on what you can build, not what you destroy."

"To those who cling to power through corruption and deceit and the silencing of dissent, know that you are on the wrong side of history, but that we will extend a hand if you are willing to unclench your fist."

Obama, 47, takes office amid national anxiety over the biggest economic downturn since the Great Depression, at a time when the nation is fighting two wars, and after a divisive 16-year period in politics. After his campaign, Obama promised to bring the country together.

The swearing in culminated a remarkable two year journey in which Obama defied expectations, first defeating Sen. Hillary Rodham Clinton in the Democratic primaries and then winning a wide electoral vote victory over Republican John McCain. He put a coalition of black, educated middle-class, Latino and young voters.

In his his first words a president, Obama sought to reassure Americans.