Washington DC - Joe Biden was sworn in as the 46th president of the United States on Wednesday, pushing a message of national unity as he takes the reins of a country reeling from a pandemic, economic turmoil and deep political divisions.

In his traditional inaugural address, seen as a key chance to set the tone of his term, Biden declared that "democracy has prevailed" as he hailed the transfer of power, despite the deadly storming of the Capitol earlier this month.

"This is America's day. This is democracy's day. A day of history and hope, of renewal and resolve," the Democrat said.

"My whole soul is in this – bringing America together, uniting our people, uniting our nation and I ask every American to join me in this cause."

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He also noted testing times ahead, including the role of the US in the world, racial injustice and the raging coronavirus pandemic.

After holding a moment of silence for the over 400,000 lives lost due to the virus in the US, he urged people to unite in the fight against the outbreak, saying the country must put politics aside.

"In the work ahead of us we are going to need each other. We need all our strength to persevere through this dark winter," he said in his first speech as president.

Biden took the oath of office in a ceremony administered by Chief Justice John Roberts just before noon, when former president Donald Trump's turbulent four-year term officially ended.

The 78-year-old became the oldest US president in history.

Moments earlier, Kamala Harris was sworn in as US vice president by Justice Sonia Sotomayor, becoming the first woman of color to hold the office.

The ceremony's star-studded line-up included the singers Lady Gaga, who belted out the US national anthem, Jennifer Lopez and Garth Brooks.

The scaled-back event on the West Front of the Capitol lacked the normal throngs of tens of thousands of people, both due to the pandemic and the recent security breaches.

More than 25,000 National Guard troops were working in Washington to secure the inauguration and the downtown of the capital city was effectively under lockdown.

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The ceremony took place without Trump, who left for Florida in the morning, breaking with tradition as he snubbed his successor's inauguration.

Biden did not mention his predecessor in his inaugural address, just as Trump did not mention Biden in his farewell remarks.

The outgoing Republican president gave a brief speech to a small crowd at Joint Base Andrews in Maryland, in which he praised his term in the presidency and did not name Biden, though he did wish the next administration good luck.

"It is my greatest honor and privilege to have been your president," Trump said. "I wish the new administration great luck and great success. I think they'll have great success, they have the foundation to do something really spectacular."

Trump, who never conceded the November presidential election, promised "to be back in some form." His political future remains unclear, but he has lost much of the social media perch he used to propel himself to the White House the first time.

The 45th president's approval ratings were at an historic low in modern history, with his popularity sharply damaged by the January 6 attack on the Capitol by a violent mob of his supporters, which left at least five people dead, including a police officer.

Trump was impeached after the riot, accused of inciting the insurrection. Social media companies banned him and he even lost support within his Republican Party, with the party's top member of Congress, Mitch McConnell, squarely blaming Trump for the riot.

Before his swearing-in ceremony, Biden went to attend Mass with his wife, Jill, at a church in Washington before the ceremony.

While Trump skipped the event, Vice President Mike Pence, McConnell and Ted Cruz, a senator who boosted unfounded claims of voter fraud, were in attendance.

Former presidents Bill Clinton, George W Bush and Barack Obama were also all on Capitol Hill for the inauguration, in the traditional show of cross-party and national unity around the celebration of a new president.