Joe Biden declared that “democracy has prevailed” after being sworn in as the 46th President of the United States, delivering his inaugural address from the same Capitol steps that had been overrun by a violent mob two week earlier.
“This is America's day. This is democracy's day. A day in history and hope, of renewal and resolve,” Mr Biden said.
It came after Kamala Harris become the first female, first black, and first Asian-American to be sworn in as vice-president.
Watch: Joe Biden becomes America's 46th president, declaring: 'Democracy has prevailed'
Mr Biden used his inauguration speech to call for unity in America. He directly addressed those who did not support him, saying: “Hear me out as we move forward”.
He added: "Take a measure of me and my heart and if you still disagree, so be it. That’s democracy, that’s America.
“I pledge this to you, I will be a president for all Americans. I will fight as hard for those who did not support me as for those who did.”
The 78-year-old president addressed the “profound” challenges facing the US, such as the coronavirus pandemic which he noted had claimed the lives of more people than the number of Americans who died in the Second World War.
“With unity we can do great things," said Mr Biden as he spoke of the “cry for racial justice” in America.
“I know speaking of unity can sound to some like a foolish fantasy. I know the forces that divide us are deep and real but I also know they are not new," he said.
“We must meet this moment as the United States of America, if we do that I can guarantee we will not fail."
Mr Biden and his wife, Jill, had arrived at the Capitol on Wednesday morning, about 90 minutes before his noon swearing-in ceremony.
They were accompanied by vice president-elect Harris and her husband, Douglas Emhoff, and were greeted by US Senator Amy Klobuchar of Minnesota.
It’s a new day in America.
— Joe Biden (@JoeBiden) January 20, 2021
Earlier on Wednesday, Mr Biden attended a mass at St Matthew’s Cathedral in Washington DC along with his wife, Jill Biden, Democratic House Speaker Nancy Pelosi and Senate Minority Leader Chuck Schumer, as well as Republican Senate leader Mitch McConnell and House leader Kevin McCarthy.
At age 78, Mr Biden will be the oldest president inaugurated.
More history will be made at his side, as Kamala Harris becomes the first woman to become vice president.
The ceremony in which presidential power is transferred unfolded at the Capitol where just two weeks ago a mob of rioters stormed the building.
With heightened security and a lack of crowds because of the threat of the coronavirus pandemic, Mr Biden’s ceremony differed significantly from the usual lavish affairs.
He was not applauded by his predecessor, who flouting tradition left the White House on Wednesday morning ahead of the inauguration, instead of accompanying his successor to the Capitol.
“Have a good life, and we will see you soon,” said Donald Trump before boarding Air Force One to fly to his beach home in Florida.
UK Prime Minister Boris Johnson and Labour leader Sir Keir Starmer joined world leaders in congratulating Mr Biden on becoming US president.
Congratulations @JoeBiden and @KamalaHarris.
The US begins a new chapter in its history, one of hope, decency, compassion and strength.
Together, our two nations can build a better, more optimistic future for our world.
— Keir Starmer (@Keir_Starmer) January 20, 2021
Mr Johnson, who had a close but sometimes difficult relationship with Mr Trump, said it was a “big moment” for the UK-US relationship.
The Prime Minister told reporters: “When you look at the issues that unite me and Joe Biden, the UK and the United States right now, there’s a fantastic joint common agenda.
“I really congratulate Joe and Kamala Harris on their achievement, on their inauguration today.
“It’s a fantastic thing for America, a step forward for the country that has been through a bumpy period.
And for us and America it’s a big moment.”
Once at the Capitol, Mr Biden was administered the oath by Chief Justice John Roberts and Ms Harris was sworn in by Justice Sonia Sotomayor.
Mr Biden and first lady Jill Biden departed the platform at the Capitol following a ceremony that included musical performances from pop stars Lady Gaga and Jennifer Lopez and country singer Garth Brooks.
US youth poet laureate Amanda Gorman read a powerful poem noting that, “while democracy can be permanently delayed, it can never be permanently denied”.
She is the youngest poet to recite at a presidential inauguration.
Later, Mr Biden will join the end of a slimmed-down inaugural parade as he moves into the White House.
Watch: What does a Joe Biden presidency in the US mean for the global economy?