WASHINGTON (AP) — Before a jubilant crowd of more than a million, Barack Hussein Obama claimed his place in history as America’s first black president, summoning a dispirited nation to unite in hope against the ‘‘gathering clouds and raging storms’’ of war and economic woe.

On an extraordinary day in the life of America, people of all colors and ages waited for hours Tuesday in frigid temperatures to witness a young black man with a foreign-sounding name take command of a nation founded by slaveholders. It was a scene watched in fascination by many millions — perhaps billions — around the world.

‘‘We gather because we have chosen hope over fear, unity of purpose over conflict and discord,’’ the nation’s 44th president said.

The presidency passed to Democrat Obama from Republican George W. Bush at the stroke of noon, marking one of democracy’s greatest gifts: the peaceful transfer of power.

But a stark transfer all the same. In one of the new administration’s first acts, Obama ordered federal agencies to halt all pending regulations until further review — this after Bush’s final weeks raised heated debate over rushing new rules into effect on the way out the door.

And even though new White House aides struggled to find offices and work intercoms, an overhauled www.whitehouse.gov Web site was running under Obama’s banner within minutes of his swearing-in. ‘‘Change has come to America,’’ it declared.

Obama plunges into his new job in earnest on Wednesday after capping inaugural festivities at a national prayer service in the morning, meeting with his economic team and Iraq advisers and welcoming a stream of public visitors into the White House while Congress gives his economic revival plan a going-over and takes up the nomination of Hillary Rodham Clinton to be secretary of state. Her confirmation has been held up for now by Republican concern over the foundation fundraising of her husband, the former president.

The new president had been buoyant and relaxed through the three days of preinaugural festivities. But he seemed somber as he stood on the Capitol steps, placed his left hand on the Bible used by Abraham Lincoln and repeated the inaugural oath ‘‘to preserve, protect and defend’’ a Constitution that originally defined blacks as three-fifths of a person. A deafening cheer went up.

‘‘What is required of us now is a new era of responsibility — a recognition, on the part of every American, that we have duties to ourselves, our nation, and the world, duties that we do not grudgingly accept but rather seize gladly,’’ Obama said. ‘‘This is the price and the promise of citizenship.’’