All 50 US states and the District of Columbia (DC) are on alert for possible violent protests this weekend, ahead of President-elect Joe Biden's inauguration on Wednesday.
National Guard troops have been sent en masse to Washington DC, to deter any repeat of last week's deadly riots.
The FBI has warned of possible armed marches by pro-Trump supporters at all 50 state capitols.
Meanwhile, the Biden team has set out plans to reverse key Trump policies.
In the hours after Mr Biden sets foot in the White House, he will embark on a blitz of executive actions designed to signal a clean break from his predecessor's administration, according to a memo seen by US media.
Among the actions:
- He will return the US to the Paris climate agreement - a global pact on cutting carbon emissions
- He will repeal the controversial travel ban on a list of mostly Muslim-majority countries
- He will make wearing masks mandatory on federal property and when travelling interstate
Although Mr Biden, like President Trump, will be able to use executive orders as a means of bypassing Congress on some issues, his $1.9tn (£1.4tn) stimulus plan announced earlier this week will need to be approved by lawmakers, as will a bill on immigration reform.
Much of Washington DC will be locked down ahead of Wednesday's inauguration, with National Guard troops deploying in their thousands.
Many streets - some miles from the Capitol, the site of deadly rioting on 6 January - have been blocked off with concrete barriers and metal fences.
The National Mall, which is usually thronged with thousands of people for inaugurations, has been shut at the request of the Secret Service - the agency charged with protecting the president.
The Biden team had already asked Americans to avoid travelling to the nation's capital for the inauguration because of the Covid-19 pandemic. Local officials said people should watch the event remotely.
Sunday is expected to also be a particular focus for protests, after posts on pro-Trump and far-right online networks called for armed demonstrations on that day.
Some militias have told their followers not to attend, however, citing heavy security or claiming the planned events are police traps.
Capitol Police confirmed on Saturday that an armed Virginia man carrying a "non-government issued credential" had been stopped and arrested at a security checkpoint on Friday with at least one gun and 509 rounds of ammunition.
But the man, named Wesley Allen Beeler, was later released from custody and told the Washington Post that he did not intend to bring a firearm into Washington, where he said he had been working with a private security firm.
"I pulled up to a checkpoint after getting lost in DC because I'm a country boy," he said. "I showed them the inauguration badge that was given to me."
The tough security measures follow a week in which Donald Trump became the first US president to be impeached twice. He now faces a Senate trial, on a charge of "incitement of insurrection" linked to the storming of the US Capitol by groups of his supporters. They did so in a bid to thwart Congress certifying Joe Biden's victory in the presidential election.
Dozens of arrests have been made in relation to the assault on the Capitol building.
Among the latest is a far-right media personality known as Baked Alaska - real name Anthime Joseph Gionet. A criminal complaint says he was arrested in Houston, Texas on Friday and charged with two federal crimes, including violent entry and disorderly conduct on Capitol grounds.
How are different states preparing?
States across the country are also taking precautionary measures, from boarding up capitol windows to refusing to grant permits for rallies.
The governors of Maryland, New Mexico and Utah have all declared states of emergency ahead of possible protests at their legislatures.
California, Pennsylvania, Michigan, Virginia, Washington and Wisconsin are among those activating their National Guards, and Texas will shut its state capitol from Saturday until after inauguration day.
According to the director of the Texas Department of Public Safety, intelligence suggested "violent extremists" could infiltrate planned protests there to "conduct criminal acts".
Virginia's Governor Ralph Northam told a news conference on Thursday: "If you're planning to come here or up to Washington with ill intent in your heart, you need to turn around right now and go home. You are not welcome here, and you're not welcome in our nation's capital. And if you come here and act out, Virginia will be ready."
Analysts believe states that saw especially hostile or protracted election battles are at most risk of violence. One of them, Michigan, has erected a six-foot fence around its capitol in Lansing.
"We are prepared for the worst, but we remain hopeful that those who choose to demonstrate at our capitol do so peacefully," the state's Police Director Joe Gasper said on Friday. The building will have an increased police presence until at least mid-February, he added.
In October, six men were arrested for allegedly plotting to kidnap and overthrow Michigan Governor Gretchen Whitmer, a Democrat. The group planned to gather about "200 men" to storm the capitol building and take hostages, investigators said.
In a further development on Saturday, Facebook said it would temporarily halt adverts for gun accessories and military kit in the US. The site had already banned adverts for guns and ammunition.
"Out of an abundance of caution, we are temporarily banning ads promoting weapons accessories and protective equipment in the US until at least January 22nd," spokesperson Liz Bourgeois told Buzzfeed.
The move came after three senators and four attorneys general urged Facebook to stop "placing profit ahead of our Nation's democracy".
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