Amid the honeycomb of offices and hidden rooms on the ground floor of a shabby brick building facing the Hudson River, activists and agitators unite for classes like “Antonio Gramsci: Revolutionary Strategy and the Historic Bloc” and talks like “Envisioning a Post-Capitalist Future.” Networks of pipes snaking along the ceiling and glimpses of exposed brick give the space a slightly industrial feel, which seems fitting for discussions on labor theory and worker exploitation.

But there is also the monthly Game Night, when regulars put down their copies of “Das Kapital” and immerse themselves in table tennis, foosball and a complicated Marxist version of Monopoly called, appropriately, Class Struggle.

In a city known for cynicism, the Brecht, which survives on donations, is a surprisingly open and idealistic place.

“We have folks who are superfamous, along with folks who are homeless,” said Kazembe Balagun, 34, outreach director of the forum for the past three years. A big, bearded, bespectacled man, Mr. Balagun (born Keith Alexander Mitchell) manages to be both bashful and effusive as he greets visitors in his sunny way. He is a child of mainline Democrats, but Mr. Balagun said he was attracted to Marxism because it offered “a way to understand the world, and understand how the world could be different.”

The forum, named for the German playwright Bertolt Brecht, began in 1975 as the New York Marxist School and had several downtown Manhattan addresses before settling in the West Street building, which once housed Serge Gym, in 2005. Over the years, Noam Chomsky has joined in forum activities, and so has Tony Kushner.

No test of ideological purity is administered at the door. Even the rare Republican is welcome, and admission to all events is either free or based on what visitors say they can afford. Working at the Brecht is part of a life of activism, Mr. Balagun said; but it can be a good time, too.

When I visited one evening in October, about two dozen youngish, fashionable people — men in button-down shirts and dress slacks, women in knee-high boots and hoop earrings — had gathered under harsh flood lamps in the forum’s circular meeting space for a panel discussion, “Child Welfare From the Crack Era to the Age of Obama.”

As the panelists spoke, a group of well-dressed ladies nodded and hummed their approval.

“We’re a society that puts capital before children,” one panelist said.

“Mm-hmm,” the women responded.

“Every woman that has a child is not a mother,” another panelist later said.

“Mm-hmm,” they crooned again.

After the talk, people chatted in the lobby, and Mr. Balagun shared amusing tales: like the one about how the supermodel Tyra Banks once ran into the Brecht to use the bathroom, and how the singer Bjork once rode by on the handlebars of a bicycle.

But when the audience had dispersed into the night, a small cadre of loyalists gathered around a table for something more entertaining: poker. Despite the temptation, I politely excused myself.

As I tried to slink away, Anoush Dertaulian, a forum volunteer, stopped laying out chips and guided me, for one last lesson, to a shrine honoring Hampig Sassounian — an Armenian immigrant imprisoned in California for murder. Ms. Dertaulian said he was a political prisoner. “You’ve never heard of him,” she added, hurrying back to the game. She was right.

While Mr. Balagun waved me out the front door, I imagined Marx’s ghost floating in the hazy light of the evening, watching over the poker players. Behind his famous thicket of a beard, I could almost see a grin.