Joe Biden Jr has been sworn in as the 46th president of the United States.
Mr Biden promised to marshal a spirit of national unity to guide the country through one of the most perilous chapters in American history.
“This is America’s day, said Biden. "Democracy has prevailed," he added after taking the oath of office from Chief Justice John Roberts.
Donald Trump - who has not formally conceded the presidency to Mr Biden - snubbed the inauguration ceremony, in a departure from longstanding precedent.
Millions of Americans watched from home as Chief Justice John Roberts administered the 35-word oath of office to Biden, moments before noon, when he formally inherited the powers of the presidency.
“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,” he said, promising to dedicate his “whole soul” to rebuilding a country ravaged by disease, economic turmoil, racial inequality and political division.
In his speech he also added: “We will be judged, you and I, by how we resolve these cascading crises of our era,” Biden said in his address. “Will we rise to the occasion, is the question. Will we master this rare and difficult hour?
"Will we meet our obligations and pass along a new and better world to our children? I believe we must.”
Former presidents Barack Obama and wife Michelle, George W Bush and wife Laura and Bill Clinton and wife Hillary attended the inauguration. Former vice-president Mike Pence and wife Karen also attended the event that was snubbed by Trump.
Hours after being sworn in, Biden was expected to begin undoing what his chief of staff described as “the gravest damages” of his predecessor’s legacy. Biden will sign 15 executive orders, as well as a series of memorandums and decrees from the Oval Office, according to his top policy advisers.
He has plans to rejoin the Paris climate accords, end the effort to leave the World Health Organisation, repeal a travel ban on several predominantly Muslim countries, revoke the permit for the Keystone XL pipeline and extend a pause on student loan payments and a federal moratorium on evictions and foreclosures.
The inauguration took place at the US Capitol. There was extra-tight security after the building was stormed by violent pro-Trump protesters in a deadly riot on 6 January.
Kamala Harris becomes first woman and first Black and Asian-American vice-president
Kamala Harris was sworn in as vice-president ahead of Mr Biden. She is the first woman - and the first black and Asian-American person - to serve in the role.
She was sworn in by Sonia Sotomayor, the first Latina Justice on the US Supreme Court.
Her election to the second-highest office in the executive branch comes five decades after the passage of the Voting Rights Act that sought to enfranchise Black Americans.
The vice president was rarely absent from the campaign trail and throughout the transition, signalling the hands-on approach the incoming administration has pledged.
Ms Harris developed a personal relationship with Mr Biden during her term as California’s attorney general, when she became close with then-Delaware attorney general Beau Biden, the president’s son. He died following a brain cancer diagnosis in 2015.
Vice-President Harris swore in three new Democratic senators on Wednesday, leaving the upper chamber of Congress evenly split between the two main parties. This will allow the vice-president to act as a tie-breaker in key votes.
Performances from Lady Gaga and Jennifer Lopez
The ceremony featured performances from Lady Gaga who sang the national anthem and Jennifer Lopez who performed "This Land is Your Land." A minute and a half into the song, she switched to "America the Beautiful" before speaking a line of the Pledge of Allegiance in Spanish and belting out a part of her own hit "Let's Get Loud".
Garth Brooks performed Amazing Gracing, asking Americans to join him.
Amanda Gorman, the US' ’ first national youth poet laureate, delivered a poetry reading to the nation, in a historic first for the 22 year-old, who was elected to the position in 2017 by UrbanWord and the Library of Congress.
Gorman called for unity and togetherness.
Biden’s inauguration brings to a close one of the most volatile transitions in modern history.
An evening concert at the Lincoln Memorial in the city will be hosted by Tom Hanks and will feature Bruce Springsteen, John Legend, Jon Bon Jovi, Justin Timberlake, and Demi Lovato.
Mr Trump first president not to attend inauguration in 150 years
Mr Trump was the first president not to attend his successor's inauguration since 1869. He left the White House early on Wednesday, and flew to the nearby Andrews Air Force base.
In his farewell address at the base, he highlighted what he regarded as the successes of his presidency. "What we've done has been amazing by any standard," Mr Trump said.
Trump's place in the spotlight is far from over. The US Senate is expected to put him on trial soon, following his record second impeachment by the House of Representatives for allegedly inciting the Capitol riot.
Through adversity Biden has persevered
Nearly half a century after he was sworn in as one of the nation’s youngest senators, he became the oldest president to take the oath of office, at 78.
A veteran of Washington first elected to the Senate in 1972, where he served until becoming vice-president under Barack Obama in 2009, Biden enters the White House with one of the deepest résumés in American political history, experience he will rely as he faces what he called “this time of testing”.
Loss and recovery have marked his long career in public service. His first wife and his daughter were killed in a car accident days after his election to the Senate. In 2015, he buried his eldest son, Beau, who died of brain cancer.
Meanwhile, Biden has proposed a $1.9 trillion Covid-19 relief package that Democrats are eager to pass quickly, with an impeachment trial for the former president looming in Congress.