As Americans began voting on Tuesday, vigilantes who fear the election will be rigged and voting rights advocates who fear voter intimidation were keenly watching, The News York Times reported.
On one side were groups like the Oath Keepers, one of dozens of rightwing and militia groups responding to Donald J. Trump's warnings about a stolen election, the daily said.
The organization has issued a nationwide "call to action" to its members, urging them to go "incognito" to polling stations on Election Day to "hunt down" instances of fraud.
On the other side are more than 100 civic and legal groups, claiming at least 10,000 volunteers, and perhaps many more, the Times said.
They plan to deploy at polling places to watch for signs of voter intimidation and other roadblocks to free voting.
"Election officials and observers say they are hoping for an orderly final day of voting, but they are girding for the possibility of fights, intimidation and, perhaps, worse," the report said.
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There is also fear of Election Day hacking, perhaps by foreign interests.
"I would say this is the most frightening election period I can remember in my adult life," Richard L. Hasen, an election law expert and professor at the University of California, Irvine, was quoted as saying.
For months, Trump has warned of widespread voter fraud, stoking suspicion and unrest among his supporters.
While he has presented no evidence for these claims - and experts say election fraud is exceedingly rare - his warnings have resonated among his followers.
Fifty-six percent of likely voters for Trump have little or no confidence in the election's fairness, according to the Pew Research Centre.
Trump has repeatedly urged his supporters to monitor polling stations. The campaign's website includes a sign-up form for "election observers" to "Help Me Stop Crooked Hillary From Rigging This Election!"
Some early voters in a number of states have reported isolated episodes of intimidation, the Times said.
Voting rights advocates say they were especially concerned about attempts to keep African-Americans and Latinos from voting.
A coalition of more than 100 civil and voting rights groups operating under the Election Protection Coalition banner will send at least 10,000 volunteers to polling places in 29 states.
Civil rights groups are also concerned about self-appointed election monitors carrying firearms.
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