A federal judge blocked construction of a 91-room luxury hotel on the Georgetown waterfront yesterday, ruling that the National Park Service illegally rescinded a height restriction for development next to Rock Creek.

U.S. District Judge Barrington D. Parker said the construction would diminish views from Rock Creek and Potomac Parkway and defeat the purpose of a 1941 restriction to protect major elements of the national park system here, including the C&O Canal, from intrusive development.

Parker's decision is the latest chapter in the long-running saga over development of the Potomac River shoreline in Georgetown and is a major victory for a small band of environmentalists who have fought development efforts on the narrow strip of land between the Whitehurst Freeway and the river.

Washington Harbour Associates, a development group that includes Western Development Corp. of Georgetown, is nearing completion on a massive commercial and luxury condominium complex on the waterfront known as Washington Harbour. In addition, the developers had agreed to sell the adjoining 1.2-acre tract between 30th Street NW and Rock Creek to Rosewood Hotels Inc. for construction of the hotel and an office building.

The Park Service, to facilitate that construction, rescinded the 20-foot height restriction on the property last fall, deciding that the Dallas-based hotel chain could build a 52-foot-high hotel and a 60-foot office building. In exchange, the developers agreed to give the Park Service permanent access along the riverfront and Rock Creek and donate $1 million toward development of a waterfront park west of the Washington Harbour complex.

Cornish F. Hitchcock, a lawyer for Public Citizen Litigation group who represented the four environmental groups who sued to block Rosewood's plans, said the ruling "indicates that land given for a park should remain that way and not be given up by the Park Service."

The developers, under existing zoning and the height restriction, can build a 20-foot structure on the property, but now would not be required to grant the public the permanent access along the Potomac River and Rock Creek. Nonetheless, Hitchcock said the environmental groups "suspect that some of the improvements will be made in any event," even if a lower structure is built, such as a shopping mall as the developers have suggested.

Manus J. (Jack) Fish, the Park Service's regional director here, and Robert D. Zimmer, Rosewood's president, denounced the ruling and said they will appeal it to the U.S. Circuit Court of Appeals here.

"Obviously we thought that we had a good position or we wouldn't have made the deal," Fish said. "We still think so."

The Park Service determined that the swap was worth $102,144 more for the public than the height restriction it gave up. Federal law requires that the swap be equal or favor the government.

Zimmer said, "I can't tell you how bitterly disappointed I am. I think we've had a well-planned, disciplined proposal to benefit Washington and Georgetown. We absolutely believe the Park Service acted within the law. I can't believe a small group has thrown such a monkey wrench into it. I'm going to fight this all the way."

The four groups who sued to block the hotel and office construction are the Committee of 100 on the Federal City, a group concerned with planning issues here; the Human Environment Center, a group that partly works for the preservation of inner-city open space; the Environmental Policy Institute, an environmental and energy issue lobbyist, and the Committee for Washington's Riverfront Parks, a group of community activists trying to preserve open space along Washington's shorelines.

Parker, in his 29-page ruling, said that "the history of the government's interest in obtaining the restrictive height easement is such that it was obtained for the express purpose of protecting the view from Rock Creek and Potomac Parkway."

The judge noted that John G. Parsons, a Park Service official here, said at a January 1984 hearing on the land-swap proposal that the height restriction was intended "to protect park resources."

Parker said that the Interior Department's "unwillingness to purchase the private property on the Georgetown waterfront (perhaps due to a lack of funds) . . . will not cure the legal defect inherent in the exchange."

The government contended at a hearing earlier this month that the exchange was legal under a law that permits the secretary of the interior to accept title to nonfederal property in exchange for an interest in federally owned land.

But the environmental groups argued, and Parker agreed, that another federal law takes precedence. That law states that the secretary can exchange property, but only if he does "not alienate property administered as part of the national park system in order to acquire lands by exchange."

Parker said, "The particular facts of this case compel the conclusion that the government's restrictive height easement . . . is not 'suitable for exchange.' "