U.S. 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 U.S. Capitol after the building was stormed by violent pro-Trump protesters on Jan 6.
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.