Organizers said they were prepared for skeptics.

But they did not expect the vitriol unleashed when a photo of their new, double-parked patrol cars on Fifth Avenue in Bay Ridge turned up Dec. 21 on Facebook, and later on Instagram. The hostility spread after a far-right Canadian website, Rebel Media, posted a snippet on YouTube. The ugly online comments included accusations that the group was a stalking horse for Shariah law, and worse.

“I expected some sort of, ‘Wait, what is that?’ but not, ‘Wait, what the hell is that?’ Mr. Rabah said. “There’s a big difference.”

Some in the Muslim community were equally startled, but for a different reason: The cars’ resemblance to New York City police cruisers stoked anxieties rather than allayed them.

Somia Elrowmeim , the adult education and women’s empowerment manager at the Arab American Association of New York, based in Bay Ridge, said a single misstep from the patrol could reflect poorly on the city’s entire Muslim community. She said more outreach to community leaders was essential before patrols began operating.

Until then, Ms. Elrowmeim, 34, offered this message: “We don’t want you near our community.”

The 68th Precinct, serving Bay Ridge and Dyker Heights, fielded a flurry of calls from concerned residents after the social media posts, leading police officials to hold an introductory meeting with Muslim Community Patrol members to discuss parameters: Call 911 if they encounter something suspicious, and take no enforcement action on their own. “We expect them to follow the law in general,” a Police Department spokesman said.

No date has been finalized for the start of patrols, Mr. Rabah said. Before the rollout, members will attend community board meetings to explain the patrol’s mission and to answer questions from residents. On Friday, the group met with members of the influential Arab American Association of New York, whose vice president, Habib Joudeh, had said he was not told about the patrol group until after the backlash prompted by the photo of the car.

“You have to inform people of what’s going on first,” Mr. Joudeh said.

Unlike the Shomrim, which patrols in vehicles and on foot, the Muslim patrol will operate only from patrol cars, Mr. Rabah said.