A couple of nights each week, the corner of 63rd Street and Park Avenue is not as quiet as some residents in the neighboring co-ops would have it. The offender? One of the oldest institutions on the block: the Third Church of Christ, Scientist.

The church looks a bit different these days. Its name, etched into stone above the white columns, was covered up last year. The pews have been removed and turned into wainscoting.

Two years into an arrangement between the church and a caterer, the foot traffic into the 84-year-old Georgian-style structure is light on parishioners and heavy on revelers, 40,000 of whom turned out last year for events such as law firm holiday parties, Oscar de la Renta fashion shows, and everything in between, according to numbers provided by the caterer. Guests have included President Clinton and Mayor Bloomberg.

In interviews, neighbors complain of double-parked limos, extra traffic, and the presence of people outside on smoke breaks.

“This may be better done down in Chelsea than on Park Avenue,” a resident across the way at 570 Park Ave., George Davis, said, adding that this was “the wrong part of town” for parties.

Whether the caterer, the Rose Group, can keep its lease with the church has been the subject of a lawsuit in U.S. District Court in Manhattan ever since the city tried to end the arrangement and was sued by the church. The case could turn on evidence ranging from e-mail messages between city officials, including those of Deputy Mayor Daniel Doctoroff, to the social calendars of residents of Park Avenue’s most established co-ops.

Churchgoers claim in court papers that a 20-year lease the church gave to the Rose Group is needed to prevent the dwindling congregation from having to sell the church. There are now less than a 100 congregants at the church, while in the 1940s and 1950s the Wednesday-evening testimonials at the church could attract a crowd of 1,400.

The deal was intended to “save the historic building” and raise enough money for the congregation to “perpetuate its mission and existence,” a trustee of the church, Thomas Draper, said in a court affidavit.

The Rose Group comprises a father-son team of caterers, Herbert and Louis Rose, who made their careers at the Pierre Hotel and Cipriani, respectively. It has pumped $8.5 million into the church for renovations.

At the center of the court case is a flip-flop by the city, which initially okayed, but later rejected, the church’s proposal to partner with Messrs. Rose.

After initially approving the arrangement in 2006, the Department of Buildings later determined that the catering operation was too large to be considered ancillary to the church. The arrangement violated a zoning resolution, the city claimed.

An outside lawyer for the church, Victor Kovner, provides a different explanation for the city’s about-face: the clout that the disgruntled neighbors wield with City Hall. Mr. Kovner described the neighbors in court papers as a “group of affluent, influential denizens of Park Avenue.”

In the months before the city revoked its approval for the plan, Mr. Doctoroff met with representatives and lobbyists for both the Rose Group and the nearby co-ops. The neighbors with whom Mr. Doctoroff met included a resident of 550 Park Ave., Peter Price, who is a former publisher of the New York Post, according to information from several e-mail messages to Mr. Doctoroff that are now court exhibits.

Later, several church members and Louis Rose sat down with Mr. Doctoroff to discuss the catering operation, Mr. Rose told The New York Sun.

Mr. Rose recalled telling Mr. Doctoroff, who has since left city government to run Bloomberg LP, that it would be unfair if the city were to revoke his permit.

“What if I tell you that’s too bad?” Mr. Rose recalls Mr. Doctoroff saying.

“I felt that the decision had already been made,” Mr. Rose said.

A spokesman for Mayor Bloomberg, Stuart Loeser, disputed Mr. Rose’s account, saying, “Deputy Mayor Doctoroff said nothing of the sort, and the whole purpose of the meeting was to work out a solution.”

The church’s suit alleges that the city isn’t treating the church on equal footing with several nearby nonreligious institutions, such as the Regency Hotel, the Council on Foreign Relations, the Colony Club, and the Asia Society, all of which rent space out to the general public.

The church has also subpoenaed the co-op boards at 570, 575, and 580 Park Ave., as well as one individual resident of 580 Park, Susan Relyea, who was present at the meeting with Mr. Doctoroff. A lawyer for the co-ops, Phyllis Weisberg, said the subpoenas appear to seek documents about any social events that the co-op boards know to have occurred in the building.