WASHINGTON (AP) — Stepping into history, Barack Hussein Obama grasps the reins of power as America’s first black president in a high-noon inauguration amid grave economic worries and high expectations.

Braving temperatures in the teens, hundreds of thousands of excited people descended on the heavily guarded capital city Tuesday for the first change of administrations since the terror attacks of Sept. 11, 2001. They packed the National Mall from the Washington Monument to the U.S. Capitol for a glimpse of the proceedings and, in the words of many, simply ‘‘to be here.’’

Two years after beginning his improbable quest as a little-known, first-term Illinois senator with a foreign-sounding name, Obama moves into the Oval Office as the nation’s fourth youngest president, at 47, and the first African-American, a barrier-breaking achievement believed impossible by generations of minorities.

Around the world, Obama’s election electrified millions with the hope that America will be more embracing, more open to change.

The dawn of the new Democratic era — with Obama allies in charge of both houses of Congress — ends eight years of Republican control of the White House by George W. Bush. He leaves Washington as one of the nation’s most unpopular and divisive presidents, the architect of two unfinished wars and the man in charge at a time of economic calamity that swept away many Americans’ jobs, savings and homes.

Bush — following tradition — left a note for Obama in the top drawer of his desk in the Oval Office.

White House press secretary Dana Perino said the theme of the message — which Bush wrote on Monday — was similar to what he has said since election night: that Obama is about to begin a ‘‘fabulous new chapter’’ in the United States, and that he wishes him well.

The unfinished business of the Bush administration thrusts an enormous burden onto the new administration, though polls show Americans are confident Obama is on track to succeed. He has cautioned that improvements will take time and that things will get worse before they get better.

Culminating four days of celebration, the nation’s 56th inauguration day began for Obama and Vice President-elect Joe Biden with a traditional morning worship service at St. John’s Episcopal Church, across Lafayette Park from the White House. Bells pealed from the historic church’s tower as Obama and his wife, Michelle, arrived.

The festivities won’t end until well after midnight, with dancing and partying at 10 inaugural balls.

By custom, Obama and his wife were invited to the White House for coffee with Bush and his wife, Laura, followed by a shared ride in a sleek, heavily armored Cadillac limousine to the U.S. Capitol for the transfer of power, an event flashed around the world in television and radio broadcasts, podcasts and Internet streaming.