“It grew into a kind of anchor or touchstone for many activists,” said Leslie Cagan, who was the primary organizer of a 1982 demonstration in Central Park against nuclear weapons that drew hundreds of thousands of people and who helped organize protests in 2004 during the Republican National Convention in New York. “Brecht provided an atmosphere that encouraged debate and dialogue and an exchange of ideas.”

Benjamin Shepard, a human services professor at the City University of New York, said that he was inside the center on Saturday evening, participating in a class centering on the Marxist theorists Antonio Gramsci and Georg Lukacs, when the discussion was interrupted with an announcement of the closing. Mr. Shepard said that those inside the center were stunned by the news.

“There we were, talking about the need to maintain counterinstitutions of the left,” he said. “And meanwhile the counterinstitution we were sitting in was crumbling around us.”

The forum, which is named for the German playwright Bertolt Brecht, began in 1975 as the New York Marxist School and was located in a few different spots before ending up in a building on West Street in 2005. The center recently moved to a community center in Boerum Hill called the Commons Brooklyn.

The center’s mission, according to its website, is to “create, within existing society, a counter-hegemonic culture of working people and their allies, who are capable of challenging the capitalist agenda, prefiguring new ways of thinking and of self-organization, as well as creating new ways of relating to each other and nature.”