A “Free Speech Rally” fizzled out before it could begin in on Saturday — as conservative activists ditched their own event when throngs of counter-protesters chanting anti-Nazi slogans converged en masse on downtown Boston.

The few dozen conservatives — some waving American flags and wearing Make America Great Again hats — wound up surrounded by an estimated 15,000 counter-protesters, some shouting “shame” and throwing plastic water bottles at them.

Some 500 police worked to keep the groups separated. Aerial video showed lines of police officers, some in riot gear and carrying billy-clubs, advancing to push back crowds in Boston Common.

By mid-afternoon the dozens who had gathered for the Free Speech Rally — which had publicly distanced themselves from the neo-Nazis and white supremacists that sparked violence last week in Charlottesville — had been escorted by police out of the Commons before the rally could start.

But not before numerous heated scuffles and shouting matches earlier in the afternoon, which erupted despite KKK and neo-Nazi groups steering clear.

Boisterous counter-protesters chased a man with a Trump campaign banner and cap, shouting and swearing at him, the AP reported.

Other counter-protesters quickly intervened on his behalf, helping the man safely over a fence to where the rally was to be staged.

Nearby, a shoving match broke out between an American-flag-waving older woman and a counter-protester wearing the black garb of the “Anti-fa” anti-fascist group.

Video captured the black-clad protester then trying to run off with the flag as she clung to it and ran after him. She stumbled and fell to the ground.

Event organizers invited several far-right speakers who were penned into a small area that cops sets up in Boston Common park to keep the sides separate.

“We’ve made it out of the Boston Rally safe and sound,” tweeted one of those scheduled to speak, Kyle Chapman, who has been charged with beating counter-protesters with a lead-filled cudgel at a pro-Trump rally in Berkeley in March.

“Thank you Boston PD Excellent job! Unlike Cville PD who stood down and caused violence.”

The Boston Free Speech Coalition, which organized Saturday’s rally, insisted they are opposed to the white supremacist message and violence that erupted a week ago in Charlottesville.

“We stood up for free speech and won,” tweeted another planned speaker, Joe Biggs, formerly of the right-wing conspiracy website Infowars.