Apparently, Friends of Dag Hammarskjold Plaza is no fan of milkshakes.

The community group is suing the city’s Department of Parks and Recreation in an effort to block the Original New York Milkshake Co. from operating a cafe in the park across from the United Nations.

Charging that the parks department acted in an “arbitrary and capricious manner,” the residents contend in the lawsuit that city officials were duped by unrealistic revenue projections from the Milkshake Co. and did not consider its lack of experience as a concession vendor. The eatery closed its only location in the East Village earlier this year.

The group had been hoping that Hospitality Holdings, the current operator of the Patio cafe in the plaza, would be able to keep its license which expires at the end of the month. The suit, filed last week in State Supreme Court, seeks an injunction against the concession awarded to the Milkshake Co.

The Milkshake Co. didn’t respond to requests for comment last night.

“We were shocked. We were absolutely shocked when we heard that an operator that we had never heard of would be operating the site,” the founder of Friends of Dag Hammarskjold Plaza, Anne Saxon-Hersh, said. Ms. Saxon-Hersh said the cafe is crucial to the success of the park, which is located across from the U.N. on East 47th Street between First and Second avenues.

“When this property is delinquent, it adds to the delinquency of the park,” she said, pointing to a period in the 1990s when the area was dilapidated. The plaza was renovated late in the decade, and Ms. Saxon-Hersh said the Patio cafe had been operating successfully for the last three years.

An owner of the Patio, Mark Grossich, said he was “flabbergasted” by the loss of the bid, saying it “doesn’t hold up to business scrutiny.”

The parks department said its policy is not to comment on pending litigation, but officials said the agency followed city regulations in awarding the bid.

The food is another issue.

The Milkshake Co.’s menu of $5 milkshakes and grilled cheese sandwiches is not appropriate for the “gateway to the United Nations,” Ms. Saxon-Hersh said, adding that she was looking for a restaurant with more continental fare.

The lawsuit has the support of local elected officials, including Assemblyman Jonathan Bing. The city comptroller’s office confirmed yesterday that it was investigating the residents’ claims after receiving complaints about the bid process.