Joe Biden on Wednesday began dismantling the legacy of his predecessor faster than any other incoming president in the modern era.
Aides said he was set to sign up to 15 executive orders on his first evening in the Oval Office, far more than other recent president.
They were aimed at halting or reversing some of Donald Trump's most controversial policies.
That was set to turn into a flood of actions over Mr Biden's first 10 days as he sought to implement measures without waiting for approval by Congress. Jen Psaki, Mr Biden's press secretary, said: "There are many more to come. His focus is on moving the country forward."
She added: "He is taking historic action on Day One to advance his agenda including signing 15 executive actions and asking agencies to take steps in an additional two areas."
Mr Biden wore a mask while seated behind the Resolute Desk with a stack of orders early Wednesday evening. He said there was "no time to start like today."
While his predecessor Donald Trump broke long-standing practice by skipping Mr Biden's inauguration, he did follow through on one tradition and left behind a letter for his successor.
The new Democratic president said Trump "wrote a very generous letter." But Mr Biden said he wouldn't reveal its contents until he had a chance to speak with Trump.
Mr Biden's immediate priority was the pandemic, and his first action was an executive order that all US government employees wear masks, and that face coverings become mandatory on federal property.
He has already urged all Americans to wear masks for the first 100 days of his presidency. Immigration was also a major focus with at least six of his opening salvo of executive orders addressing the issue.
One was set to end Donald Trump's ban on travel from several predominantly Muslim countries, and another would stop building of the outgoing president's wall on the US-Mexico border.
Mr Biden's order on the border wall ends a national emergency declaration which allowed Mr Trump to divert government money for its construction.
The new president was not ordering that the wall that has been built be torn down.
In addition, Mr Biden intended to immediately announce an immigration bill that would open an eight-year path to citizenship many of the estimated 11 million illegal immigrants in the US. According to aides there was also an intention to quickly show Mr Biden's determination to "re-engage in the world".
His first orders also included a move to rejoin the Paris climate accord, and a reversal of Mr Trump's decision for the United States to leave the World Health Organization.
Mr Biden also planned to make one of his first actions the nomination of a new US surgeon general, following the resignation of Jerome Adams, who was appointed by Mr Trump.
Dr Anthony Fauci, the top US infectious diseases expert, who had a fractious relationship with Mr Trump, will deliver a speech on Thursday to the WHO, setting out how the Biden administration will proceed on the pandemic.
Although Mr Biden was fulfilling his campaign pledge to rejoin the Paris climate accord "on day one" it will take 30 days for the US to be officially back in.
The new president was also instituting a temporary moratorium on new oil and gas leasing in the Arctic National Wildlife Refuge, which was authorised by Mr Trump.
He was also moving immediately to revoke a presidential permit, issued under Mr Trump, for the Keystone XL oil and gas pipeline.
Mr Biden's actions were intended to send a signal that he wanted to hit the ground running.
None of his immediate predecessors signed more than one executive order on their first day in office, instead attending celebratory inaugural balls.
Mr Trump, on his first day in 2017, signed one executive order, aimed at reversing Obamacare. Barack Obama signed none on his first day.
On Thursday Mr Biden was set to sign further executive orders relating to the pandemic, and aides said he would move "aggressively" toward safely re-open schools and businesses, including through an expansion of testing.
In the following days he will sign measures including revoking a ban on transgender military service.
On Monday he plans to issue an executive order requiring the US government to "Buy American". The next day he will issue an order against private prisons.
There will be further actions relating to climate change, immigration, refugee policies and healthcare in his first 10 days.
Mr Biden will also start measures aimed at reuniting families separated at the Mexico border under the Trump administration.
He then plans to spend February "restoring America's place in the world," according to a schedule that has been roughed out.
In a joint statement Republican members of Congress said Mr Biden's plans showed he was being dragged to the left by his own party.
They said: "The Biden administration has made it clear that on 'Day One' they will immediately cater to the far left instead of working to help all Americans and move our country forward."