With his hand on a five-inch thick heirloom Bible that has been in his family for more than a century, Biden took the oath of office administered by US Chief Justice John Roberts that binds the president to "preserve, protect and defend the Constitution of the United States."
"Through a crucible for the ages, America has been tested anew, and America has risen to the challenge," Biden said in his inaugural address. "Today we celebrate the triumph not of a candidate but of a cause: the cause of democracy...At this hour, my friends, democracy has prevailed."
Biden, 78, became the oldest US president in history at a scaled-back ceremony in Washington that was largely stripped of its usual pomp and circumstance, due both to the coronavirus and security concerns following the Jan. 6 assault on the US Capitol by supporters of outgoing President Donald Trump.
The norm-defying Trump flouted one last convention on his way out of the White House when he refused to meet Biden or attend his successor's inauguration, breaking with a political tradition seen as affirming the peaceful transfer of power.
Trump, who never conceded the Nov. 3 election, did not mention Biden by name in his final remarks as president on Wednesday morning, when he touted his administration's record and promised to be back "in some form." He boarded Air Force One for the last time and headed to his Mar-a-Lago retreat in Florida.
Top Republicans, including Vice President Mike Pence and the party's congressional leaders, attended Biden's inauguration, along with former US Presidents Barack Obama, George W. Bush and Bill Clinton.
Biden takes office at a time of deep national unease, with the country facing what his advisers have described as four compounding crises: the pandemic, the economic downtown, climate change and racial inequality. He has promised immediate action, including a raft of executive orders on his first day in office.
After a bitter campaign marked by Trump's baseless allegations of election fraud, Biden struck a conciliatory tone, asking Americans who did not vote for him to give him a chance to be their president as well.
"To overcome these challenges to restore the soul and secure the future of America requires so much more than words. It requires the most elusive of all things in a democracy: unity," he said. "We must end this uncivil war that pits red against blue, rural versus urban, conservative versus liberal. We can do this - if we open our souls instead of hardening our hearts."
The ceremony on Wednesday unfolded in front of a heavily fortified US Capitol, where a mob of Trump supporters stormed the building two weeks ago, enraged by his false claims that the election was stolen with millions of fraudulent votes.
The violence prompted the Democratic-controlled US House of Representatives to impeach Trump last week for an unprecedented second time.
Thousands of National Guard troops were called into the city after the siege, which left five people dead and briefly forced lawmakers into hiding. Instead of a throng of supporters, the National Mall on Wednesday was covered by nearly 200,000 flags and 56 pillars of light meant to represent people from US states and territories.
"Here we stand, just days after a riotous mob thought they could use violence to silence the will of the people, to stop the work on our democracy, to drive us from this sacred ground," Biden said. "It did not happen; it will never happen. Not today, not tomorrow, not ever."
'SOUL OF AMERICA'
Biden's inauguration is the zenith of a five-decade career in public service that included more than three decades in the US Senate and two terms as vice president under former President Barack Obama.
But he faces calamities that would challenge even the most experienced politician.
The pandemic in the United States reached a pair of grim milestones on Trump's final full day in office on Tuesday, reaching 400,000 US deaths and 24 million infections - the highest of any country. Millions of Americans are out of work because of pandemic-related shutdowns and restrictions.
Biden has vowed to bring the full weight of the federal government to bear on the crisis. His top priority is a $1.9 trillion plan that would enhance jobless benefits and provide direct cash payments to households.
But it will require approval from a deeply divided Congress, where Democrats hold slim advantages in both the House and Senate. Harris was scheduled to swear in three new Democratic senators late on Wednesday, creating a 50-50 split in the chamber with herself as the tie-breaking vote.
Biden will waste little time trying to turn the page on the Trump era, advisers said, signing 15 executive actions on Wednesday on issues ranging from the pandemic to the economy to climate change. The orders will include mandating masks on federal property, rejoining the Paris climate accord and ending Trump's travel ban on some Muslim-majority countries.
Although Biden has laid out a packed agenda for his first 100 days, including delivering 100 million COVID-19 vaccinations, the Senate could be consumed by Trump's upcoming impeachment trial, which will move ahead even though he has left office.
The trial could serve as an early test of Biden's promise to foster a renewed sense of bipartisanship in Washington.
Trump issued more than 140 pardons and commutations in his final hours in office, including a pardon for his former political adviser, Steve Bannon, who has pleaded not guilty to charges that he swindled Trump supporters as part of an effort to raise private funds for a Mexico border wall.
But Trump did not issue preemptive pardons for himself or members of his family, after speculation that he might do so.
