The Mall will be shuttered to the public for nearly a week, federal officials announced Friday, the latest in a series of unprecedented security measures that will all but prevent most people from viewing the inauguration in person on Wednesday.

The extraordinary measure was just one that was announced Friday as the heavy safety cordon around the nation’s capital tightened. Virginia will shut down four major bridges leading into downtown D.C. ahead of Inauguration Day. And defense officials authorized 4,000 more National Guard troops, swelling the total expected in the city to 25,000.

State leaders were also hurrying to protect government buildings, while mobilizing additional police officers and National Guard troops ahead of expected right-wing protests this weekend and Inauguration Day. Some governors worried violence could shift from D.C. to less fortified state capitols. Maryland Gov. Larry Hogan (R) declared a disaster Friday ahead of the inauguration and asked for federal aid.

D.C. Mayor Muriel E. Bowser (D) and representatives from the Secret Service, Federal Emergency Management Agency and National Park Service advised the public at a Friday news conference that they are still prepared for unprecedented security threats in the coming days. They urged — as they have repeatedly — for people to stay away from D.C. They are hoping to avoid a repeat of the Capitol siege.

“We cannot allow a recurrence of the chaos and illegal activity that the United States and the world witnessed last week,” said Matt Miller, head of the Washington Field office for the Secret Service, which is heading inauguration security.

The heightened security measures conjured a stark image for Inauguration Day: a celebration of democracy mostly devoid of citizens.

Every four years people from all over the country flock to the nation’s capital for the inauguration. But two weeks after the heart of the country’s democracy was attacked by a mob of pro-Trump rioters, there will be no dancing on the Mall. No throngs pressing to get a view of the presidential motorcade on Pennsylvania Avenue. And few tourists mugging for photos in oversized T-shirts of the new president.

On Wednesday, National Guard troops in the city will likely outnumber spectators for the presidential swearing-in.

As part of an agreement with the Secret Service, Virginia announced the bridge closures Friday evening.

From Tuesday at 6 a.m. to Thursday at 6 a.m., the Virginia State Police will shut down the Theodore Roosevelt Bridge, Arlington Memorial Bridge, Interstate 395 Bridge, and 14th Street Bridge to both vehicular and pedestrian traffic.

The closures show the lengths that federal and local law enforcement are going to prevent armed extremists from corralling in Washington in the wake of violent threats targeting lawmakers ahead of the inauguration.

Virginia Gov. Ralph Northam (D) and members of the Virginia delegation — including Democratic Sens. Mark R. Warner and Tim Kaine, and Democratic Reps. Don Beyer, Gerald E. Connolly and Jennifer Wexton — said the bridge closures were intended to ensure the transfer of power will be “as peaceful as possible.”

“The 2021 Presidential Inauguration Ceremony will see the strongest Capital-area security response in history,” they wrote in a joint statement. “We worked together to push for a response that balances protecting public safety in a manner commensurate with available intelligence about threats without going too far.”

The announcement came as more National Guard troops streamed into D.C. on Friday. They will come from all 50 states, three territories and D.C., defense officials said.

Numerous states also have activated guardsmen to either protect civilians and state facilities this weekend, or to hold on standby status in case violence erupts. Among the states expecting to have guardsmen on the streets are Ohio, North Carolina, Pennsylvania and Washington state.

Fences were going up around state capitols in Sacramento and Phoenix, where some mailboxes were removed as jittery officials worried they could be used to hide explosive devices. Armed guards were stationed around Georgia’s Capitol.

Michigan state officials will shutter the state legislature from Tuesday through Thursday after receiving threats of violence deemed credible.

In the District, National Guard members have staffed an increasingly fortified perimeter that encompasses the Capitol, White House and portions of downtown. Clad in their camouflage uniforms and helmets, they are mostly carrying rifles without magazines of ammunition in them, in an apparent attempt to avoid accidents. The magazines can be carried in pouches on their tactical vests or in pockets, and attached to the guns within seconds if needed.

A D.C. National Guard spokesman, Capt. Edwin Nieves Jr., said the decision on how guardsmen are handling their weapons is made in coordination with the agencies the National Guard is supporting, which include police and the Secret Service.

“Guardsmen are armed and equipped commensurate with the missions they have been asked to do,” Nieves said.

None of the security measures were expected to alter the inaugural ceremony itself, which had already been pared back because of concerns about the pandemic. Nevertheless, officials called the security buildup “unprecedented” and historians reached decades back to find parallels for the coming inauguration.

“This may be the most unusual inauguration in American history. Maybe not the most consequential, but the most unusual,” President-elect Biden said at an event Friday.

The Mall has never been shuttered during an inauguration since the ceremonies began being held on the West Front of the Capitol in 1981, said Jim Bendat, an inaugural historian and author of “Democracy’s Big Day.”

Bendat said Biden’s inauguration could be the smallest since 1945. That year, Franklin D. Roosevelt decided to hold the swearing-in at the White House because he was in ill health and the United States was fighting World War II, Bendat said. Only about 1,000 people attended.

“It’s a pretty stunning development, but it’s also understandable given what happened on January 6th,” Bendat said of the closure of the Mall. “It’s really sad. What we had on January 6th was a real assault on democracy. Now, this is a real symbolic assault on democracy.”

Bendat said the Mall has long been the spot where thousands have gathered to watch and participate in the peaceful transition of power from one administration to the next.

Crowds have regularly numbered in the hundreds of thousands. Bendat said the largest crowd on the Mall was believed to be for President Barack Obama’s first inauguration in 2009, which some estimates put at 1.8 million.

But not this year.

People attempting to enter the fortified downtown area will be subject to searches. Bowser asked residents to sign up for alerts about closures, which may change over the next few days.

While most of downtown has turned into a fortress meant to keep people away, there will be slices of land available for demonstrations, despite Bowser’s request that the interior secretary deny all permits in downtown Washington. The Park Service said at a news conference Friday that it will screen and escort two groups of up to 100 permitted demonstrators each to areas near the U.S. Navy Memorial and John Marshall Park on Inauguration Day.

Jeff Reinbold, superintendent of the Mall and Memorial Park, said, “We’ve had the chance to talk to them and make sure they’re able to scale their First Amendment demonstrations to the size we have available.”

Maryann Tierney, regional administrator with FEMA, said that she has deployed regional teams to D.C., Maryland and Virginia following the president’s emergency declaration around the inauguration. Her teams have pre-staged ambulances and moved food and other resources closer to the District in case need arises.

Leaders also said they are prepared to redirect resources away from downtown D.C. if demonstrators are scared away by their security posture.

“We do recognize that because we have such a robust and hardened perimeter . . . there is the potential for people to go elsewhere, whether it is back to their state capitals or other parts of the city,” Miller said.

He added that the National Guard Bureau would be ready to respond to other state capitals “in a timely and appropriate fashion” if there is intelligence detecting threats outside of the city.

Acting D.C. police chief Robert J. Contee III said that his officers would be “on hand to ensure a peaceful day, both for inaugural events and around our great city.”

The regional and federal team is preparing for threats as early as this weekend. Federal officials had previously warned of the possibility of armed demonstrations in D.C. and all 50 state capitals between Saturday and Wednesday.

Miller said his agency is investigating and talking with a number of extremist groups about plans for imminent demonstrations in D.C.

“There’s a great deal of chatter, and it’s what you don’t know that we’re preparing for,” he said. No one raised their hand to say they’re coming, “but we’re preparing as if they are.”

Bowser said they are having a “daily download” with the FBI to get that information.

Instead of vast throngs heading to D.C. as is customary for the days leading up to inauguration, some were canceling trips and D.C. residents were planning on fleeing the city or hunkering down.

Mollie Davis, a 20-year-old from St. Mary’s, Md., spent the past year watching and cheering for the only politician she has seen stutter like she does. Biden’s success on the national stage put her at ease, she said, and made her feel for the first time like nothing, not even her speech disorder, would stand in the way of her dreams of a career in politics.

Jan. 20 was supposed to be a day for Davis to celebrate that hope on the Mall with her dad. But she began to reconsider her plans to watch the swearing-in ceremony in person when the number of coronavirus cases began to surge in the area. Last Wednesday’s insurrection was the final straw. She canceled her trip downtown out of fear of armed protesters descending once again on the nation’s capital.

“It would have been cool personally to see a person who stutters sworn in in person, but at the same time, I am concerned for a lot of people’s safety, so it’s for the best,” she said. “I’ve made my peace with that.”

Two Northwest D.C. fathers, Tim Mazzarell and Jon Groteboer, said on a recent day at a playground, they would take their respective families away from the capital for the inauguration.

Mazzarell is going to a family home in Rehoboth, Del., Groteboer to a family cabin in West Virginia.

“It seems safer in the woods,” Groteboer said.

“I was shocked about what happened last week,” Mazzarell said. “It got so unsettling to be around.”

Jeff Brooke, 58, another D.C. resident, said he will treat this Inauguration Day like a “snow day” by stocking up on food and water, and staying indoors in case businesses close and another curfew is enacted.

Asked about whether D.C. will go back to normal after the extreme security measures on display in the city, Bowser says D.C. will go to a “new normal” after the insurrection last Wednesday.

“We all have to think about a new posture, we certainly have to think about a new posture in the city,” she said.

Michael Brice-Saddler, Perry Stein, Lauren Lumpkin, Tim Craig and Paul Schwartzman contributed to this report.