Officials in Boston, Massachusetts, are bracing for possible confrontations as rival protests converge on the city centre on Saturday.
The organisers of the conservative “Free Speech Rally” said they would not offer a platform to racism or bigotry.
A counter-protest is also planned, with some 11,000 people saying on Facebook they plan to attend.
Tensions are high after violent demonstrations in Charlottesville, Virginia, last weekend turned deadly.
Police were investigating reports that some radical counter-protesters might plan to throw acid at rally-goers and even police, a law enforcement official told the Boston Globe.
The city police commissioner said he had never seen so many people “almost looking for confrontation” ahead of the competing demonstrations.
“I just think the rhetoric has really brought this to a different level, and that’s what we’re worried about,” Commissioner William Evans told a news conference on Friday.
The organisers of the “Free Speech Rally” said that “misinformation in the media” was “likening our organization to those that ran the Charlottesville rally”.
“While we maintain that every individual is entitled to their freedom of speech and defend that basic human right, we will not be offering our platform to racism or bigotry,” the group wrote on a Facebook page dedicated to the event. “We denounce the politics of supremacy and violence.”
The list of speakers for the free speech event has changed multiple times in previous days. At times it has included speakers that some have associated with the far-right.
The so-called “Antifa” left-wing activist group has said it plans to attend as well.
“If anything gets out of hand, we will shut it down,” Boston Mayor Marty Walsh said on Friday morning.
A free speech rally in the city in May saw duelling protests, shouting and foul language, according to local media reports from the time.