Kamala Devi Harris made history at the stroke of noon on Wednesday becoming the first vice president of Indian descent and Joseph Robinette Biden made a call for unity as he became president of a nation in the throes of a devastating Covid-19 pandemic and the "raging fire" of national division.

The daughter of Shyamala Gopalan became the first Indian American, the first woman, and the first African American vice president, putting her a heartbeat away from the presidency.

Biden declared after being swearing-in, "Democracy has prevailed."

And to the world, he said, "We will engage with the world again… we will not lead merely by the example of our power, but by the power of our example."

Symbolically, Supreme Court Justice Sonia Sotomayor, the first Latina to serve on the court, gave her the oath of office before Biden.

Her husband Doug Emhoff held two bibles on which she swore, one of them that belonged to Regina Helton, whom she has called her "second mother", and another to Thurgood Marshall, the first African American supreme court justice.

Biden in his speech noted that 108 years ago protesters had stood there to block women from demanding the right to vote, and said, "And today we mark the swearing of the first woman in American history elected to national office, Vice President Kamala Harris."

Next, Biden swore on a hefty family bible that his wife Jill Biden held. Chief Justice John Roberts gave him the oath of office.

The transfer of power went off peacefully as they assumed the presidency and vice presidency as mandated by the Constitution, without the feared disruptions.

Soon after the ceremony, Biden went into the president's office in the Capitol to sign a series of orders nominating key members of his cabinet.

Donald Trump, who boycotted the ceremony, had already left Washington for his new home in Florida.

Two weeks after a mob of Trump supporters had attacked the Capitol, they stood on its west frontage to be sworn-in.

The 25,000 National Guard called in to protect them ceremony outnumbered the guests. Unlike in the past, people were kept away because of the Covid-19 crisis and security considerations.

A carefully controlled guest list of a few hundred that included former Presidents Barack Obama and George W Bush, former Vice President Mike Pence and other luminaries were at hand.

The national anthem was sung by Lady Gaga, a pop singer and the national pledge of alliance was recited by a woman firefighter, who also displayed it in sign language.

Senator Amy Klobuchar, who had been one of the rivals of Biden for the Democratic Party nomination, acted as the emcee for the ceremony. Biden's speech was put together by a team led by Vinay Reddy, his director of speechwriting.

"Few people in our nation's history have been more challenged or found a time more challenging or difficult than the time we're in now," Biden said focusing on the Covid-19 pandemic and the raging disunity marked by the riot at that very spot.

"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," he said.

"I promise you, I will fight as hard for those who did not support me as for those who did," he said.

Recalling Abraham Lincoln's speech when he made the proclamation abolishing slavery, Biden said, "Today, on this January day, my whole soul is in this: bringing America together, uniting our people, uniting our nation."

He also quoted Augustine, "a saint in my church", who "wrote that a people was a multitude defined by the common objects of their love" and asked, "What are the common objects we as Americans love, that define us as Americans?

Biden and Harris are to visit the tomb of the Unknown Soldier and after that they will be led in a formal procession to the White House.

There Biden will sign a series of executive orders, mostly to reverse Trump's actions. They are expected to include rejoining the Paris Climate Agreement.