US police fired pepper spray Saturday after counter-demonstrators accused them of protecting "Straight Pride" advocates who support President Donald Trump, and refused to let officers re-open a road.
The unrest came after the counter-protesters and "Straight Pride" group -- considered homophobic extremists by their opponents -- staged dueling rallies in Boston.
There was no serious violence during the rallies themselves, which involved groups of several hundred.
But after the "Straight Pride" group had dispersed, the counter-protesters accused police of protecting "Nazis," chanted "shame on you" and formed a human chain which police tried to get through, an AFP photographer said.
Officers fired pepper spray and made several arrests.
Earlier outside City Hall, in what is one of America's most liberal cities, people from the two camps stood close enough to yell in each other's faces and in a few instances threw coffee cups and soil at each other.
The demonstrations came as tensions simmer between leftists and white nationalists across the United States, and critics of Trump say his rhetoric has fueled extremism.
Black-clad riot police with batons and gas masks stood by in formation, ready to snuff out any trouble.
Many in the "Straight Pride" group wore capes or other attire bearing the US stars and stripes flag.
Some also bore Israeli flags, while the counter-protesters hurled chants of "Nazi" and "Free Palestine."
An AFP photographer saw the remains of several US flags that had been burned.
Before the more serious unrest, a couple of counter-demonstrators scuffled with police and threw dirt and eggs at them.
A group calling itself Super Happy Fun America organized the "Straight Pride" parade in response to the gay pride parades that take place, jamming the streets of US cities every year.
Critics say the "Straight Pride" organizers are white-supremacists whose intent is to bait members of the LGBT community in Boston.
The marshal of the "Straight Pride" rally was Milo Yiannopoulos, a prominent and provocative member of the far-right movement in America who has been banned from several social media sites. He also happens to be gay.
"My fellow homosexuals have been embarrassing me for decades," he told AFP. "Finally I found my crowd."
"Honestly, as a gay man, I have been embarrassed and attacked by my fellow homosexuals for most of my life, and these people feel like home."
He added that he stands for traditional Christian family values -- "even if I don't always live them."
An "oppressed majority"
Rachel Domond, an organizer of the counter-demonstration, said she turned out to "stand up against this hatred that is here in Boston, and nationally."
Trump's rise, she told AFP, had made people like the white supremacists feel "empowered to say those statements and be more public about it."
Two weeks ago, a far-right rally and counter-demonstration by anti-fascist protesters in the city of Portland, Oregon passed with no major incident amid a heavy police presence.
Super Happy Fun America's website says Saturday's march was designed to "spread awareness of issues impacting straights."
It quotes the group's president, John Hugo, as describing heterosexuals as "an oppressed majority."
Massachusetts was the first US state to legalize same-sex marriage.
But the parade had a clear political slant too, as participants walked alongside pro-Trump floats. The group's website displays a doctored photo of Trump holding a sign saying #GreatToBeStraight.
A "Straight Pride" rally held the previous weekend in Modesto, California attracted only a few dozen people and about 250 counter-protesters, according to local newspaper The Modesto Bee.
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