Officials at the Frick Collection knew their plan to build a six-story addition to a beloved, landmark, jewel-box museum would draw detractors. But leaders of that Fifth Avenue museum say they didn’t expect it to get so intense so fast.

Since the museum announced its expansion in June, more than 2,000 critics of the plan have signed a petition put together by a consortium of preservation groups that have created a website and given themselves a name: Unite to Save the Frick.

Now, the group says it has found evidence that the museum, whose plan needs city approval, is going back on a promise made in its original landmark review roughly 40 years ago to make permanent a garden by the noted landscape architect Russell Page that is to be destroyed in the expansion.