Thousands of counter-protesters marched through central Boston on Saturday, chanting anti-Nazi slogans and waving signs condemning white nationalism.
They protested ahead of a rally being staged by conservative activists a week after a Virginia demonstration turned deadly.
Police Commissioner William Evans said Friday that 500 officers, some in uniform, others undercover, would be deployed to keep the two groups apart on Saturday.
Boston's Democratic mayor, Marty Walsh, and Massachusetts' Republican governor, Charlie Baker, both warned that extremist unrest would not be tolerated in the city famed as the cradle of American liberty.
Organisers of the event, billed as a Free Speech rally, have publicly distanced themselves from the neo-Nazis, white supremacists and others who fomented violence in Charlottesville on August 12.
A woman was killed at that Unite The Right rally, and scores of others were injured, when a car ploughed into counter-demonstrators.
But opponents feared that white nationalists might show up in Boston anyway, raising the spectre of ugly confrontations in the first potentially large and racially-charged gathering in a major US city since Charlottesville.
Events are planned around the country, in cities including Atlanta, Dallas and New Orleans.
Mr Walsh greeted counter-protesters on Saturday morning outside Reggie Lewis Centre in the city's Roxbury neighbourhood.
Counter-protesters from Black Lives Matter and other groups denouncing racism and anti-Semitism are planning to march from there to the Common, and another group plans to rally on the steps of the Statehouse overlooking the sprawling park.
The permit issued for the rally on Boston Common came with severe restrictions, including a ban on backpacks, sticks and anything that could be used as a weapon.
The permit is for 100 people, though an organiser has said he expected up to 1,000 people to attend.
The Boston Free Speech Coalition, which organised the event, said it has nothing to do with white nationalism or racism and its group is not affiliated with the Charlottesville rally organisers in any way.
"We are strictly about free speech," the group said on its Facebook page. "We denounce the politics of supremacy and violence."
But the mayor pointed out that some of those invited to speak "spew hate." Kyle Chapman, who described himself on Facebook as a "proud American nationalist", said he will attend.
Black Lives Matter said Friday that members from around the US planned to march Saturday in Boston.
Mr Walsh said the city would do whatever is necessary to head off violence initiated by either side. "If anyone gets out of control, at all, it will be shut down," he said.
"We will not tolerate any misbehaviour, violence or vandalism whatsoever," said Mr Evans.
Dating to 1634, Boston Common is the nation's oldest city park. The leafy central park is popular with locals and tourists and has been the scene of numerous rallies and protests for centuries.