Democrat Joe Biden was sworn in as the 46th president of the United States on Wednesday, ending the tumultuous four-year term in the White House of Republican Donald Trump.

Biden, 78, took the oath of office from Supreme Court Chief Justice John Roberts at a solemn ceremony at the US Capitol which was snubbed by the outgoing president and took place in the shadow of a raging coronavirus pandemic.

Before Biden, Kamala Harris was sworn in as US vice president, becoming the first woman, the first Black person and the first Asian American to hold the office.

The scaled-back ceremony in Washington that saw Biden become the oldest US president in history had been largely stripped of its usual pomp and circumstance, due both to the coronavirus and security concerns following the January 6 assault on the US Capitol by Trump supporters.

See: The 46th US president: Joe Biden

Following his oath of office, Biden celebrated his incoming administration not as a celebration of a candidate but a victory for US democracy, saying more work must be done to heal the nation.

“The will of the people has been heard, and the will of the people has been heeded. We’ve learned again that democracy is precious and democracy is fragile. At this hour, my friends, democracy has prevailed," Biden said in his inaugural speech. “Today we celebrate the triumph, not of a candidate, but of a cause. The cause of democracy.”

And then he pivoted to challenges ahead, acknowledging the surging virus that has claimed more than 400,000 lives in the US. Biden looked out over a capital city dotted with empty storefronts that attest to the pandemic's deep economic toll and where summer protests laid bare the nation's renewed reckoning on racial injustice.

“We have much to do in this winter of peril, and significant possibilities: much to repair, much to restore, much to heal, much to build and much to gain,” Biden said. “Few people in our nation's history have more challenged, or found a time more challenging or difficult than the time we're in now.”

He vowed to defeat political extremism and domestic terror. The US faces “a rise of political extremism, white supremacy, domestic terrorism, that we must confront, and we will defeat”, he said in his first speech.

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.”

Biden, in his third run for the presidency, staked his candidacy less on any distinctive political ideology than on galvanising a broad coalition of voters around the notion that Trump posed an existential threat to American democracy. Biden did not mention Trump by name in the early moments of his inaugural address but alluded to the rifts his predecessor had helped create.

“I know the forces that divide us are deep and they are real. But I also know they are not new. Our history has been a constant struggle between the American ideal that we all are created equal and the harsh, ugly reality of racism, nativism, fear, demonisation that have long torn us apart,” Biden said. “This is our historic moment of crisis and challenge, and unity is the path forward and we must meet this moment as the United States of America.”

Trump departs

Trump left the White House with his wife Melania just after 8am (1300 GMT) and went by helicopter to a sendoff event at Joint Air Force Base Andrews, where he promised supporters “we'll be back in some form” and extolled his administration's successes before flying off to Florida.

Top Republicans, including Vice President Mike Pence, were not there to see him go. Biden arrived at the Capitol just before 10:30am for his inauguration after a visit to church, where he was joined in a show of unity by the two most senior Republicans in Congress: Senator Mitch McConnell and House of Representatives Minority Leader Kevin McCarthy.

With only a small number of attendees present, Biden took the oath of office around noon, placing his hand on an heirloom Bible that has been in the Biden family for more than a century.

Trump flouted one last convention on his way out. His refusal to attend his successor's swearing-in breaks with more than a century and a half of political tradition, seen as a way of affirming the peaceful transfer of power.

The president did, however, leave a customary note for Biden in the Oval Office, according to a White House official, though it was not yet known what it said.

Biden's running mate, Harris, the daughter of immigrants from Jamaica and India, became the first Black person, first woman and first Asian American to serve as vice president after she was sworn in by US Supreme Court Justice Sonia Sotomayor, the court's first Latina member.

Tense atmosphere

The tense atmosphere in Washington evoked the 1861 inauguration of Lincoln, who was secretly transported to Washington to avoid assassins on the eve of the Civil War, or Roosevelt's inaugural in 1945, when he opted for a small, secure ceremony at the White House in the waning months of World War II.

As Biden and Harris were sworn in, Vice President Mike Pence, standing in for Trump, sat nearby as Lady Gaga, holding a gold microphone, sang the National Anthem accompanied by the US Marine Corps band.

Biden oversaw a Pass in Review, a military tradition that honours the peaceful transfer of power to a new commander in chief. Later, Biden, Harris and their spouses were to be joined by former presidents Bill Clinton, George W. Bush and Barack Obama — all of whom attended the ceremony along with their wives — to lay a wreath at the Tomb of the Unknown Soldier at Arlington National Ceremony.

Still later, Biden was to join the end of a slimmed-down inaugural parade as he moves into the White House. Because of the pandemic, much of this year's parade was to be a virtual affair featuring performances from around the nation.

In the evening, in lieu of the traditional glitzy balls that welcome a new president to Washington, Biden will take part in a televised concert that also marks the return of A-list celebrities to the White House orbit after they largely eschewed Trump. Among those in the lineup: Bruce Springsteen, Justin Timberlake and Lin-Manuel Miranda.

"I protested 45's inauguration, and I wanted to be here when he left," said Raelyn Maxwell of Park City, Utah. "And I wanted to celebrate the new president." She brought a bouquet of roses she hoped to toss to Harris and some champagne to toast the occasion.

Trump's second impeachment trial could start as early as this week. That could test the ability of the Senate, poised to come under Democratic control, to balance impeachment proceedings with confirmation hearings and votes on Biden's cabinet choices.

Biden was eager to go big early, with an ambitious first 100 days that includes a push to speed up the distribution of Covid-19 vaccinations to anxious Americans and pass a $1.9 trillion virus relief package. On day one, he'll also send an immigration proposal to Capitol Hill that would create an eight-year path to citizenship for immigrants living in the country illegally.

He also planned a 10-day blitz of executive orders on matters that don't require congressional approval — a mix of substantive and symbolic steps to unwind the Trump years. Among the planned steps: rescinding travel restrictions on people from several predominantly Muslim countries; rejoining the Paris climate accord; issuing a mask mandate for those on federal property; and ordering agencies to figure out how to reunite children separated from their families after crossing the border.