Snipers on rooftops and officials wearing bulletproof vests are among extreme measures at some US voting locations, according to a senior election official.
Barbed-wire fencing has also been put up at several places that saw trouble in the wake of the 2020 presidential vote, Tammy Patrick told Sky News.
It comes as Philadelphia's district attorney, Larry Krasner, told troublemakers to stay away, warning: "We're not playing. F around and find out."
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Ms Patrick, chief programme officer at the National Association of Election Officials, said 40% of workers had already experienced threats or abuse.
There are fears officials could be at risk as votes are cast and counted, or that violence could break out if Donald Trump loses and disputes the result.
"Election officials have had to take some extreme measures to make sure they have protected not only themselves and their staff, but the literal ballots that contain the voices of their voters," said Ms Patrick.
She said some staff had resorted "to implementing bulletproof vests" while counting centres in places such as Maricopa County, Arizona, and Las Vegas, Nevada, had put up barbed wire.
"There are snipers on top of buildings around some of this country's election offices. These are extraordinary times," Ms Patrick told Sky News.
Metal fences were also being put up on Monday near the White House in Washington DC, not far from the infamous storming of the Capitol building after the last election.
The Maricopa County tabulation centre - in downtown Phoenix - was a flashpoint for rigged election conspiracy theorists four years ago and security has now been beefed up.
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County Sheriff Russ Skinner said his department was on "high alert" with drones monitoring activity around polling stations and snipers and other assets ready if needed.
"There will be zero tolerance on anything related to criminal activity," added Mr Skinner.
Arizona and Nevada are two of the seven key swing states expected to tip the balance and decide who becomes president.
Concerns of unrest have ratcheted up due to the extremely close nature of the contest and Donald Trump's claims - without evidence - that election cheating is already taking place.
He's also repeatedly told supporters he can only lose if Democrats cheat and has refused to say whether he will accept the results.
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Pennsylvania is another swing state - and carries more weight than the others due to its 19 electoral college votes. On Monday officials there gave a blunt warning to would-be troublemakers.
"Anybody who thinks its time to play militia, F around and find out," warned Philadelphia District Attorney Larry Krasner.
"Anybody who thinks it's time to insult, to deride, to mistreat, to threaten people, F around and find out," he added.
"We do have the cuffs, we do have the jail cells, we do have the Philly juries and we have the state prisons.
"So if you're going to try to turn an election into some form of coercion, if you're going to try to bully people, bully votes or voters, you're going to try to erase votes, you're going to try any of that nonsense, we're not playing. F around and find out. That's what we're going to do."
The worst scenario and the one most likely to spark trouble is Trump losing and refusing to concede defeat, according to Peter Simi, a sociology professor who has researched threats against public officials.
He told Reuters unrest could be "dispersed, diffuse events across multiple locations" rather than a repeat of the mass storming of the US Capitol building.
(c) Sky News 2024: Snipers on roofs and US election staff in bulletproof vests, says official