Mayor Zohran Mamdani held a major town hall meeting Thursday evening in Brooklyn about improving bus service, one of his signature campaign issues.

At the event, the mayor’s political allies, the Democratic Socialists of America, set up a table alongside transit experts and city agency staff.

A leading good-government group found it to be an inappropriate mix of politics and government.

“I found it to be a shocking image, frankly,” Grace Rauh, director of Citizens Union, said. “Every one of us who was involved at all in politics in any way, shape or form knows that the DSA is primarily a political organizing shop.”

Norman Siegel, a civil rights lawyer, said it’s the first time he’s seen something like this.

“If the DSA received quote, ‘the favorite status position,’ that would be not appropriate,” he said. “I think it’s important and especially raises the question in the background of the mayor himself and his relationship to the DSA.”

Mayor Mamdani’s spokeswoman, Dora Pekec, said DSA members were not there for politics, but rather, their subject-matter expertise. Those in attendance were part of the organization’s transit working group, akin to Transportation Alternatives or Riders Alliance.

“These organizations provided a variety of resources and perspectives on transit advocacy for New Yorkers,” Pekec said.

The difference, however, between the DSA and those transit groups is that the DSA is officially a social welfare organization — a 501(c)(4) in the IRS code. Its members can lobby and engage in politics, “so long as that is not its primary activity,” according to the IRS website.

The DSA is also officially not a political party.

Gustavo Gordillo, co-chair of the New York City chapter of the DSA, said in a statement that the DSA does extensive community organizing and social welfare work.

“We were one of many non-profit community organizations that tabled at the event with the goal of giving New Yorkers the tools to fight for a stronger bus system — our area of expertise,” he said.

Mamdani’s conservative critics say it crossed a red line.

“They are just making it all-inclusive, which is very Marxist. It comes out of the theory of Communism,” Councilwoman Vickie Paladino, a Queens Republican, said. “You kind of blend together government and politics as one.”

“It just seems to me that they’re still getting special treatment and I think we have to talk about a higher standard,” Councilman David Carr, the Council Republican leader, said.