Thousands of antiracism protesters marched in downtown Boston on August 19, where conservative activists appeared to cut short a rally one week after a Virginia demonstration turned deadly.
Reports said dozens of people who had gathered in the Boston Common park left less than an hour after the Free Speech Rally was getting under way.
TV cameras showed police vans escorting participants away, and angry counterprotesters scuffled with armed officers trying to maintain order, AP reported.
Organizers of the Free Speech Rally had said they would not give a platform to racism or bigotry.
Many counterprotesters wore stickers with the face of Heather Heyer, who died when a car was driven into a crowd of counterprotesters at the August 12 far-right and neo-Nazi rally in Charlottesville, Virginia.
Boston police said 27 arrests had been made, mostly for disorderly conduct and some for assault and battery.
President Donald Trump praised the Boston police on Twitter, writing, "Looks like many anti-police agitators in Boston. Police are looking tough and smart! Thank you."
Rallies were held elsewhere in the United States, including in Dallas and Austin, Texas; Atlanta; Laguna Beach, California; and New Orleans, with authorities in most locations reporting that antiracism activists outnumbered right-wing marchers.