The turning of the US Capitol into a fortress of fences, concrete barriers and security checkpoints continued Monday as thousands of National Guard, federal and local law enforcement officers remained vigilant for any threats before the inauguration of President-elect Joe Biden.

Meanwhile, law enforcement officials are vetting airplane passengers at airports across the nation, putting any who have been identified among the violent protesters at the Capitol on Jan 6 on a "no fly list".

And the FBI and US Army, prompted by some current and former police officers and military personnel who joined the attack on the Capitol, screened National Guard troops in the capital for any extremist elements among the thousands of troops.

To get a glimpse of how the jittery capital is gripped by the fear following the deadly Jan 6 attack on the US Capitol, the Capitol complex temporarily locked down for about an hour Monday morning during a rehearsal for the Jan 20 event.

The lockdown was ordered, and some participants were evacuated after a fire in a homeless encampment roughly a mile away sent a plume of smoke into the air and caused security concerns.

The rehearsal resumed not long afterward, after the fire was quickly extinguished.

Biden has urged his supporters not to travel to Washington for his inauguration on Wednesday, which President Donald Trump has announced he won't attend as he continues to claim the election was stolen from him.

William Banks, distinguished professor emeritus at the Syracuse University College of Law in New York, said that only once in US history, just after the Civil War, has a departing president not attended the inauguration.

"The ceremony will also be dramatically affected by the pandemic, and by the extraordinary security necessitated by the attack on the Capitol on Jan 6," he said.

District of Columbia Mayor Muriel Bowser also asked Americans not to come to Washington for Biden's inauguration, fearing violence and the spread of the coronavirus.

"Our goals right now are to encourage Americans to participate virtually and to protect the District to Columbia from a repeat of the violent insurrection experienced at the Capitol and its grounds on Jan 6," Bowser said at a news conference.

The traditional parade of military units and bands on Pennsylvania Avenue that draws hundreds of thousands of people and is reviewed by the new president and vice-president has been replaced by a "virtual" parade.

According to the National Park Service's list of inauguration permit applications it has received through the end of January, only one permit, which was still being processed, was explicitly pro-Trump.

Journalists covering the inaugural also were preparing for any violence, with CNN reporting that news organizations were giving staffers gas masks, helmets and body armor.

At least nine journalists were physically assaulted, at least five were arrested and at least four had equipment damaged while covering the storming of the Capitol, officials said.

Monday was a national holiday in observance of the birthday of the civil rights icon Martin Luther King, but the memorial in the capital is closed to the public through Thursday.

The National Mall, the Washington Monument and others were inaccessible; bridges were shut down and Metro stations shuttered. Streets from the Capitol where the inauguration of the 46th president will take place to the White House are closed, and some residents had to show identification to get home.

Ken Cuccinelli, Homeland Security acting deputy, told 60 Minutes that National Guard members in the nation’s capital swore an oath to the Constitution and will do whatever it takes to keep Americans safe.

"We're going to complete our jobs. There's not a stand-down. We have a statutory mission we’re going to perform under all circumstances. And I think that hypothetical is not going to happen. It's unimaginable," Cuccinelli said.

Pentagon officials said that 15,000 National Guard members from all 50 states and three territories had arrived in Washington by Saturday, and that number would reach as many as 25,000 by Wednesday to secure the inauguration.

"The massive show of force in Washington and the smaller displays in many state capitals are warranted and were necessary," said Cal Jillson, a political scientist and historian at Southern Methodist University.

Jillson noted that demonstrations around the nation over the pre-inaugural weekend were decidedly muted, and he believed that they largely will be the same on Wednesday as well.

"Once Biden is inaugurated, and tensions begin to abate, it will be important to get the barriers down as soon as is responsibly possible," he added.

Agencies contributed to this story.