Hundreds of union workers gathered in lower Manhattan Saturday to chant, cheer and wave signs in a show of unity ahead of a Supreme Court hearing that could have a profound effect on organized labor.

On Monday, the Supreme Court will consider Janus vs. AFSCME — a controversial legal challenge to a longstanding precedent that protects the ability of public employee unions to represent their members and even nonmembers, and to speak out on matters of public interest.

Workers from both the public and private sector flooded Foley Square early Saturday.

They were joined by a long list of elected officials, including Gov. Cuomo and Mayor de Blasio as well as State Comptroller Thomas DiNapoli.

One attendee came all the way from Memphis, Tenn., to speak at the rally.

Baxter Leach, who marched in Memphis as a striking sanitation worker more than 50 years ago along with Dr. Martin Luther King , galvanized the crowd with a few simple words.

“I want to tell you all. To get what you want you’ve got to stand up. You can’t sit down,” the civil rights icon said, his face projected on a jumbotron screen across the wide public square in front of the city’s court houses.

Gov. Cuomo called the rallying workers “the heart and soul” of the country.

“You see labor going backwards all across this country. In New York it’s a different story. In New York you see labor going forward. We want more fairness, more justice,” he said.

“We know what’s going on in this country … We have a federal administration that has declared itself an enemy of the working men and women of this nation. They have declared a war on the middle class and on the labor movement,” the governor told the cheering crowd.

The coalition of unions, economic justice and civil rights groups sponsored the rally — which was echoed at 10 other major cities around the country.

Organized loosely around the theme of #unrigthesystem, the main focus of the day was the possible pending doom of a Supreme Court ruling in Janus vs. AFSCME.

The crux of the case rests on the legality of the “agency fee,” which public employee unions in 22 states, including New York, charge workers who are represented by those unions — even if they officially decline membership.

Full union members pay union dues — while non-members pay the “agency fee,” which covers only contract-related union functions such as negotiations and enforcement, including grievance procedures.

Longstanding legal precedent allows unions to charge fees to workers who benefit from contracts they negotiate — but that may change depending on the Supreme Court arguments Monday.

Mayor de Blasio joined the Saturday rally and declared New York “the biggest union town” in America.

“ We are proud of that … We want to be a place where everyone who works hard is treated with dignity and lives a decent life,” he said.

“In D.C. ….. President Trump and his cabinet of millionaires and billionaires are trying to take away your rights,” he added, calling out the Republican party and the billionaire conservative Koch brothers who have helped finance the Janus vs. AFSCME case.

“Are we going to let them do it? Are there more of us than there are of them? And now we fight. The 1% has gotten away with it for too long,” the mayor told the spirited crowd.

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