On the cusp of becoming president, Joe Biden pressed Monday for unity, while President Donald Trump remained secluded in the White House at the center of a capital inundated with troops and security barriers.

Biden marked the Martin Luther King, Jr. holiday with a trip from his home in Delaware to pack food bags for charity in Philadelphia -- a gesture symbolizing his call for Americans to come together after four divisive years.

"Service is a fitting way to start to heal, unite, and rebuild this country we love," Biden said in a video marking the occasion.

But the 78-year-old Democrat's fervent appeals for optimism and healing -- which are also set to dominate his inauguration ceremony at noon on Wednesday -- are running up against the hard reality of multiple crises.

Covid-19 is out of control, vaccine distribution is stumbling, and economic recovery remains in the balance.

And after Trump refused for more than two months to accept the results of November's presidential election, the country is seething with division and anger. When Biden takes the oath of office at noon on Wednesday, he will face a city under the protection of more than 20,000 National Guard soldiers.

Checkpoints and large zones closed to ordinary citizens mean there will be only a smattering of guests. Similar lockdowns have been imposed at state capitol buildings around the country where local authorities fear provocations from right-wing groups ahead of the inauguration.

A brief security scare on Monday near Congress sparked an evacuation of the site where Biden will take the oath.

The acting defense secretary said, meanwhile, that the military and FBI was vetting the National Guard troopers, who carry automatic weapons, in case any of them posed a threat.

"While we have no intelligence indicating an insider threat, we are leaving no stone unturned in securing the capital," the official, Christopher Miller, said.

In a culturally significant mark, the legendary country singer Garth Brooks said that he was joining the musical line up at Biden's ceremony, stressing this was "not a political statement, this is a statement of unity."

Lady Gaga and Jennifer Lopez are already set to perform.

 Trump mulls pardons

Trump, who has still not congratulated Biden or invited him for the traditional tea visit in the Oval Office, has been largely out of the public eye since his supporters rampaged through Congress on January 6, triggering his historic second impeachment a week later.

His final Gallup poll as president on Monday showed him exiting with 34 percent approval, his record low. His overall average of 41 percent since taking office is also the lowest for any presidency's approval rating since Gallup began measuring in 1938.

According to US media, one of Trump's final actions could be announced Tuesday at the latest: scores of pardons for convicted criminals.

Speculation is mounting over whether Trump will take the unprecedented and legally murky step of issuing himself and his children, who work as campaign and White House advisors, preemptive pardons.

According to CNN and other outlets, Trump has a list of about 100 people he will grant clemency.

After what The New York Times reports has been an intense lobbying effort, these are expected to be a mix of white collar criminals and people whose cases have been championed by criminal justice activists.

More controversial possible pardons that have been the subject of speculation for months would be for the likes of Edward Snowden, Julian Assange and Trump's influential advisor Stephen Bannon.

If Trump gave himself or his children a pardon -- something currently not expected, according to latest US reports -- that would ensure a politically explosive finale to one of the most polarizing presidencies in US history.

A self pardon might also harden anger at Trump among Republicans in the Senate, which is expected to start an impeachment trial soon.

 Inauguration snub

Trump, the first president to lose reelection since George H.W. Bush was replaced by Bill Clinton, is skipping Biden's inauguration -- the first ex-president to snub his successor in a century and a half.

On Wednesday, he'll travel to his Mar-a-Lago golf club residence in Florida, departing the White House early in order to benefit from full presidential travel privileges up to the last minute.

Marine One will take him from the White House to Joint Base Andrews to catch Air Force One -- the presidential plane that will no longer be his to use from noon.

According to a Bloomberg report, Trump is organizing a military sendoff for himself at Andrews, watched by a crowd of invitees.