Tens of thousands of anti-racism protesters are converging on a “Free Speech Rally” in the US city of Boston that features far-right speakers.
They have surrounded a fenced-off area on Boston Common, where the rally is taking place, and are being held back by police barriers.
Rally organisers said they would not offer a platform to racism or bigotry.
Tensions are high after violent demonstrations in Charlottesville, Virginia last weekend turned deadly.
The BBC’s Aleem Maqbool at the scene says those who have turned up for the conservative rally are confined to the bandstand area on Boston Common.
Crowds of anti-racism protesters are totally surrounding the bandstand but are being kept some distance away by barriers, he adds.
Many wore stickers with the face of 32-year-old Heather Heyer, who died when a car was driven into a crowd of counter-protesters at last Saturday’s violence in Charlottesville.
Another anti-racist protest was being held about two miles away and those protesters are marching towards the common.
Police were investigating reports that some radical counter-protesters were planning to throw acid at rally supporters and even police, a law enforcement official told the Boston Globe.
Speaking ahead of the competing demonstrations, the city police commissioner said he had never seen so many people “almost looking for confrontation”.
“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 organisation 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 Saturday’s free speech event has changed multiple times in previous days. At times it has included speakers who 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.
The violence in Charlottesville began with a protest and counter-protest over the planned removal of a statue of Confederate commander General Robert E Lee.
In the aftermath, Confederate statues across America have come under renewed scrutiny.
Duke University in North Carolina removed a statue of Robert E Lee from its chapel entrance in the early hours of Saturday morning, following vandalism earlier in the week.
University President Vincent Price said the decision was made for safety reasons and “above all to express the deep and abiding values of our university.”