Joe Biden has been sworn in as the 46th US president, ending one of the most dramatic political transitions in American history.
“Democracy has prevailed,” he said after taking the oath of office from Chief Justice John Roberts.
Outgoing President Donald Trump, who has not formally conceded to Mr Biden, is not attending the ceremony.
The new president has announced a raft of executive orders aimed at reversing Mr Trump’s key policies.
Vice-President-elect Harris was sworn in ahead of Mr Biden – becoming the first woman and the first black and Asian-American person elevated to serve in a role a heartbeat from the presidency.
The inauguration is taking place at the US Capitol. There is extra-tight security after the building was stormed by violent pro-Trump protesters in a deadly riot on 6 January.
Some 25,000 National Guards are protecting the inauguration ceremony, which is missing the traditional hundreds of thousands of spectators because of the coronavirus pandemic.
Mr Trump left the White House for the last time as president shortly after 08:00 (13:00 GMT). He boarded a helicopter, flew to the nearby Andrews Air Force base, and has now arrived in Florida.
He is the first president not to attend his successor’s inauguration since 1869.
How is inauguration day unfolding?
In his inaugural address, Mr Biden said it was a day of “history and hope”. “My whole soul is in putting America back together again,” he added.
Highlighting a message of unity after the turbulent years of the Trump presidency, he promised to be a leader “for all Americans” – including those who voted against him.
Among those attending the ceremony are three of his predecessors: Barack Obama – under whom Mr Biden served for eight years as vice-president – Bill Clinton and George W Bush.
Outgoing Vice-President Mike Pence is also attending. He skipped Mr Trump’s farewell military salute event at Andrews base.
The ceremony includes musical performances by Lady Gaga – who sang the national anthem – as well as Jennifer Lopez and Garth Brooks.
An evening concert at the Lincoln Memorial in the city will be hosted by Tom Hanks and include Bruce Springsteen, John Legend, Jon Bon Jovi, Justin Timberlake, and Demi Lovato.
Earlier on Wednesday Mr Biden, 78, attended Mass at a cathedral in Washington – along with four Roman Catholic congressional leaders, both Democrats and Republicans.
What is Trump doing?
In his address at Andrews Air Force base the outgoing president highlighted what he regarded as the successes of his presidency. “What we’ve done has been amazing by any standard,” Mr Trump said.
The 74-year-old flew off to begin post-presidential life at his Mar-a-Lago golf club in Palm Beach.
In his last hours, Mr Trump granted clemency to more than 140 people, including his former adviser Steve Bannon, who had been facing fraud charges.
The political drama surrounding Mr Trump 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.
On Tuesday, the Senate’s Republican leader, Mitch McConnell, said the mob had been provoked by Mr Trump and fed lies.
What will Biden do on his first day?
Mr Biden has set out a flurry of executive orders. In a statement on Wednesday he said he would sign 15 orders after he is sworn in. They will:
- Reverse Mr Trump’s withdrawal of the US from the Paris climate accord
- Revoke the presidential permit granted to the Keystone XL Pipeline, which is opposed by environmentalists and Native American groups
- Revoke Trump policies on immigration enforcement and the emergency declaration that helped fund the construction of a Mexican border wall
- Bring about a mask and distancing mandate for federal employees and in federal buildings, and a new White House office on coronavirus
- End a travel ban on visitors from some, mainly Muslim, nations
Other orders will cover race and gender equality, along with climate issues.
Mr Biden’s vice-president will swear 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.
Mr Biden’s legislative ambitions could be tempered by the slender majorities he holds in both the Senate and House of Representatives.
On Tuesday, Mr Biden delivered a speech in his home state of Delaware, telling reporters “these are dark times… but there’s always light”, before heading to Washington.
In the evening, he and Ms Harris led a tribute at the reflecting pool in front of the Lincoln Memorial to the 400,000 Americans lost to Covid-19.
They were almost alone on the National Mall, where some 200,000 flags have been planted to represent the crowds who will be absent at Wednesday’s inauguration.
What’s the mood like in Washington?
Some 25,000 National Guard troops are guarding the Capitol, White House and National Mall, which are also protected by a ring of steel made up of barricades and tall fencing.
Ahead of Mr Biden’s arrival in the city, 12 National Guard members were removed from the presidential inauguration security mission after they were found to have ties with right-wing militia groups or posted extremist views online.
There was no threat to Mr Biden, officials said.
How can I watch the inauguration?
We will bring you all the latest developments, with analysis from reporting teams in Washington at the Capitol and on the National Mall.
•Online: Follow the latest updates and live video on the BBC News website and @BBCNorthAmerica. You can also stay up to date on our BBC News Facebook account and on Instagram
•Television: The BBC’s Katty Kay in Washington DC presents live coverage of the key events around the inauguration on BBC One from 16:00-18:00 GMT and on BBC News Channel (UK only) and BBC World (Outside UK only) from 15:30-19:00 GMT
•Radio: The BBC World Service has special radio coverage on Outside Source 16:00-18:00 GMT