Updated at 5:12 p.m. ET
The National Mall, where millions of people have gathered to mark historic events in Washington, D.C., was closed to the public late Friday morning, as officials announced a string of security measures meant to foil any attempts to derail next week's inauguration of President-elect Joe Biden.
The National Park Service began a "temporary public closure" of the National Mall late Friday morning, citing a request from the U.S. Secret Service. The agencies say they're worried that pro-Trump extremists are planning new violence, building on last week's insurrection at the U.S. Capitol.
"The unprecedented nature of the recent civil unrest at the U.S. Capitol coupled with the real and substantial threat of violence and unlawful behavior poses an unprecedented public safety and security challenge," the National Park Service said on Friday.
With the National Mall closure, the public will be barred from entering an area some 2 miles in length from the Capitol complex to the Potomac River; officials say the Mall will be off-limits through "at least" Thursday – the day after Biden is inaugurated.
Another attempt to block gatherings in the area comes from Facebook, which says it will stop users from creating new events close to the Capitol, White House or state capitols, and will remove existing ones that violate Facebook policies.
Many streets are also being closed off, including the Memorial Bridge that runs from the Lincoln Memorial in D.C. across the Potomac River to Arlington National Cemetery in Virginia. Other bridges entering the city could also be closed, though the timing and details are not yet certain, officials said.
Two areas along Pennsylvania Avenue will remain accessible to a limited number of demonstrators, with up to 100 people allowed to gather in each zone.
"They will be met by U.S. Park Police, escorted through magnetometers and then taken to these areas so that they can exercise their First Amendment rights," said Jeff Reinbold, the NPS superintendent of the National Mall and Memorial Parks.
Federal officials, Washington Mayor Muriel Bowser and others gave new details about their security plans at a briefing Friday. They urged people to enjoy the inauguration from home and to follow it online rather than in person.
Once a security perimeter is fully in place, any vehicle that enters will have to do so through a checkpoint, where it will be "searched for explosives, weapons and other prohibited items," said Matt Miller, head of the Secret Service's Washington field office.
Discussing needs to secure the rest of Washington and other areas that could be targeted, Miller said, "we can't create a fortress and allow the rest of the city to suffer in services" from fire, emergency and police agencies.
"It's all hands on deck for our entire agency," said the Metropolitan Police Department's acting chief, Robert Contee. "Because our entire agency has responsibility for the entire city."
Bowser acknowledged that while she agrees that the security measures, including a new fence around the Capitol complex, are necessary and prudent, she isn't happy that large chunks of her city now look like militarized zones.
"I'm sad about it, I have to tell you, that it looks that way," she said. "I'm committed to making sure that we get our city back."
But there is a special responsibility, the mayor said, to ensure "a peaceful transition of power in our country."
The National Guard did not have a representative at the briefing. When asked what Guard members' rules of engagement will be, Miller of the Secret Service said the National Guard is "printing rules of engagement cards for each of its soldiers deployed on this exercise."
Miller added that he believes that Guard members will be following the same guidelines as the agencies they're working alongside, such as the U.S. Capitol Police.