US President Joe Biden has begun to undo some of Donald Trump's key policies, hours after being sworn in.
"There is no time to waste when it comes to tackling the crises we face," he tweeted as he headed to the White House following his inauguration.
President Biden signed 15 executive orders, firstly to boost the federal response to the coronavirus crisis.
Other orders reverse the Trump administration's stance on climate change and immigration.
Mr Biden set to work at the Oval Office having been sworn in earlier on Wednesday as the 46th president of the United States.
The inauguration was unlike any other due to coronavirus restrictions, with few present to witness the oaths and ceremonies.
Donald Trump - who has still not formally conceded the presidency to Mr Biden - snubbed the event in a departure from longstanding precedent.
"Democracy has prevailed," President Biden said after taking the oath of office with Chief Justice John Roberts.
Delivering a message of unity after the turbulent Trump years, he promised to be a president "for all Americans" - including those who voted against him.
Three of his predecessors attended the ceremony: Barack Obama - under whom Mr Biden served for eight years as vice-president - Bill Clinton and George W Bush, as well as Mr Trump's vice-president, Mike Pence.
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.
There was extra-tight security for the ceremony at the US Capitol after the building was stormed by violent pro-Trump protesters on 6 January.
Mr Biden and First Lady Jill Biden, together with Ms Harris and her husband Doug Emhoff, then walked down Pennsylvania Avenue to the White House, greeting friends and supporters.
The inauguration ceremony included musical performances by Lady Gaga - who sang the national anthem - as well as Jennifer Lopez and Garth Brooks.
Amanda Gorman, America's first-ever National Youth Poet Laureate, recited her work The Hill We Climb.
A 90-minute televised evening concert entitled "Celebrating America" was staged at the Lincoln Memorial in the city. Hosted by Tom Hanks, it featured Bruce Springsteen, John Legend, Jon Bon Jovi, Justin Timberlake, and Demi Lovato and culminated in a spectacular fireworks display.
What orders has Biden signed?
President Biden "will take action - not just to reverse the gravest damages of the Trump administration - but also to start moving our country forward," a statement detailing the executive orders said.
A series of measures will be enacted to tackle the coronavirus pandemic which has claimed more than 400,000 lives in the US.
There will be a mandate to wear masks and practise social distancing on all federal government property.
A new office will be set up to co-ordinate the response to the pandemic and the US will halt the process - begun by the Trump administration - of withdrawing from the World Health Organization (WHO).
The move to re-engage with the WHO was welcomed by UN Secretary-General Antonio Guterres who said it was "absolutely critical" for a more co-ordinated global response, his spokesman Stéphane Dujarric said.
Mr Biden has also pledged to make the fight against climate change a top priority of his administration.
He signed an executive order beginning the process of rejoining the 2015 Paris climate agreement, from which Mr Trump formally withdrew the US last year.
Mr Biden's climate envoy, former US Secretary of State John Kerry, tweeted that the commitment set "a floor, not a ceiling" for America's climate leadership and urged international co-operation ahead of the UN Climate Change Conference (Cop26) in Glasgow in November.
Mr Biden has also revoked the presidential permit granted to the controversial Keystone XL Pipeline, which environmentalists and Native American groups have fought for more than a decade.
The move will be discussed when Mr Biden makes his first phone call to a foreign leader - Canadian Prime Minister Justin Trudeau - on Friday, White House press secretary Jen Psaki said.
The privately financed pipeline - estimated to cost about $8bn (£5.8bn) - would carry about 830,000 barrels of heavy crude a day from the oil sands of Alberta, in Canada, to Nebraska.
Barack Obama vetoed a bill approving construction of the pipeline in 2015 but the decision was overturned by President Trump.
On immigration Mr Biden has revoked the Trump administration's emergency declaration that helped fund the building of a wall along the Mexican border and also ended a travel ban on some majority-Muslim countries.
Other orders cover race and gender equality.
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