WASHINGTON >> Law enforcement authorities, responding to threats of violence before the Jan. 20 presidential inauguration, will deploy up to 15,000 National Guard troops to the nation’s capital and set up checkpoints around the city to avoid the botched response that helped rioters overrun the Capitol last week.

Sixteen groups — some of them armed and most of them hard-line supporters of President Donald Trump — have registered to stage protests in Washington, prompting deep concern among federal officials about an event that has historically been a packed celebration of American democracy. With coronavirus cases soaring and the deadly siege of the Capitol still fresh, the leaders of the District of Columbia, Maryland and Virginia issued a joint statement asking Americans to stay away from the inauguration of Joe Biden and instead tune in virtually.

Despite the increasing alarm, Biden’s inaugural committee said he was determined to make an outdoor appearance at the event to call on a divided nation to come together at a time of political and public-health crisis. The inauguration’s theme is “America United.”

Biden also plans to visit Arlington National Cemetery with three living presidents — Bill Clinton, George W. Bush and Barack Obama — adding another challenge for federal law enforcement authorities.

“I’m not afraid of taking the oath outside,” Biden said Monday.

But the inauguration of the 46th president could echo the first inauguration of Abraham Lincoln, whose procession to the half-built Capitol was surrounded by heavily armed cavalry and infantry troops marching through a city on the brink of civil war.

Complicating the security effort further, the head of the Department of Homeland Security, Chad F. Wolf, whose department includes the Secret Service, which is leading inauguration security, announced Monday he would be resigning at midnight Monday. Pete Gaynor, the administrator of the Federal Emergency Management Agency, will take on the role of acting secretary, Wolf said in a letter to employees.

“This inauguration is going to look differently than previous inaugurations, I think we all know that,” said Michael Plati, the Secret Service special agent in charge leading security planning for the inauguration, who referred to lessons learned from last Wednesday.

About 6,000 National Guard troops from six states have already arrived in Washington, Gen. Daniel Hokanson, the chief of the National Guard Bureau, said Monday; by this weekend, that number is expected to have grown to 10,000.

At the same time, Defense Department officials have expressed concern that some of the protesters who stormed the Capitol are former military. While the Defense Department has not announced a specific search for deployed National Guard troops with sympathies for the pro-Trump protesters, officials said they are reviewing photos and videos from the protests.

“We do not tolerate extremists in our ranks,” Pentagon spokesperson Jonathan Hoffman told reporters.

Beyond the Capitol building, the Secret Service is establishing a “healthy, layered buffer” with vehicle checkpoints, metal detectors and additional security screenings to prevent another deadly siege, Plati said.

“Obviously the Capitol is a significant event,” he said. “We are sensitive to that. We constantly are evaluating our security plans. There’s always lessons that can be learned after an event of that nature.”

To avoid the breakdowns in coordination among law enforcement agencies that left badly outnumbered Capitol Police overwhelmed last week, Plati said the Secret Service would be working out of a “multiagency coordination center” with other law enforcement leaders to make quick decisions about the movements of any protesters and make additional deployments.

“We’re going to create a bubble that is safe and secure,” Plati said.

With far-right extremists continuing to plot on online platforms, one senior Pentagon official called the security situation “unprecedented.”

Concerns are not limited to Jan. 20.

Flyers circulated in encrypted WhatsApp, Signal and Telegram groups over the weekend calling for an “armed march on Capitol Hill and all state capitols” at noon Jan. 17. The flyers, which include the instruction to “come armed at your personal discretion,” also appeared on chat sites Gab and Parler that have attracted far-right voices. Various local militia groups in states from Pennsylvania to Florida to Michigan have posted calls on those platforms for their members to come out in force.

Another group, “Let America Hear Us, Roar For Trump,” has requested a permit from the National Park Service for a demonstration of at least 300 people in downtown Washington. The Park Service on Monday shut down tours of the Washington Monument through Jan. 24, citing “credible threats to visitors and park resources.”

Far-right extremists groups continue to plot online. Nearly 400 people had joined a private group online dedicated to what is being billed as the “Million Militia March,” an event scheduled to take place in Washington on Jan. 20. Commenters have debated bringing baseball bats and body armor. The FBI has notified local law enforcement of the potential for armed protests at all 50 state capitals, which are being organized and promoted by far-right extremist groups such as the Boogaloo movement.

Mayor Muriel Bowser of the District of Columbia has asked the Park Service to stop issuing permits for public gatherings through the inauguration. Mike Litterst, a Park Service spokesperson, said the Interior secretary, whose department includes the national parks, planned to discuss plans the matter with Bowser on Monday afternoon.

Bowser also sent a letter dated Saturday to the Department of Homeland Security requesting a disaster declaration, which would free federal funding for the inauguration. Janet Montesi, a spokesperson for FEMA, said in a statement the request is currently under review.

“This is necessary because the inauguration poses several unprecedented challenges that exceed the scope of our traditional planning processes: the COVID-19 pandemic and of course the domestic terror attack on the United States Capitol,” Bowser said during a news briefing.

“If I’m scared of anything, it’s for our democracy because we have very extreme factions in our country that are armed and dangerous,” Bowser told reporters.