Hundreds of conservative protesters hosting a “Free Speech Rally” on Boston Common clashed with counter-demonstrators from local socialist and anti-fascist groups yesterday after American flags were burned and insults were hurled across a police line and at least one face-to-face confrontation turned violent.

Protesters on both sides of the police line showed up seemingly prepared for violence, many were wearing protective sports equipment, goggles and helmets, while others carried flagpoles, umbrellas and sticks. The gathering was sparked by a “Free Speech Rally” organized by local conservative and alt-right groups.

“We’re trying to show that Boston stands against hate,” said Paul Weiskel, one of the organizers of the counter-protest, who described the day as a success. “It’s clear to the public walking through the Common that there’s plenty of people who are very uncomfortable with the rhetoric and the symbols that are being used by the alt-right.”

As many pro-Trump demonstrators worked to keep their supporters on their side of the Common, members of the Boston Democratic Socialists and the Boston Chapter of Antifa assembled in front of the Soldiers and Sailors Monument waving black flags and chanting “No Trump, no KKK, no fascist USA!”

“White supremacy is alive and well and it’s important to stand up to it,” said one counter-protester who asked to be identified as Cole P.

As police officers worked to keep the two groups apart, demonstrators using megaphones spent more than four hours shouting insults across the barrier and one self-described anarchist set a small American flag on fire. At one point a girl who said she was a Quincy High School student threw a rock at the pro-Trump side.

Tensions reached a boiling point about 1:45?p.m., when 28-year-old Salvatore Guytano Cippola, of Oceanside, N.Y., crossed a police barrier in an attempt to hand one of the counter-protesters a Pepsi — a move that appeared to be an attempt to mock a recent commercial that featured Kendall Jenner handing a cop a soda.

Cippola was at least the fourth demonstrator to try to pull off the joke.

As he approached the group, a group of counter-protesters began shouting at him and telling him to go back to his side of the Common.

During the ensuing back-and-forth, Cippola and 19-year-old Elise Hinman, of Clovis, Calif., allegedly began shoving each other and eventually Cippola, Pepsi still in hand, allegedly threw a punch that struck Hinman in the face. Police say both were arrested on a charge of affray.