The New York City Landmarks Preservation Commission yesterday rejected a proposal to build an apartment tower atop the Metropolitan Club on Fifth Avenue.

The commission also made landmarks of eight more theaters in and around Times Square - the Cort, the Embassy 1, the 46th Street, the Golden, the Helen Hayes, the Mark Hellinger, the Hudson and the Imperial - bringing to 21 the number of houses designated in the theater district.

The Metropolitan Club proposal was rejected by a vote of 7 to 0, with two abstentions. The landmarks panel praised the design for the new building, by the architects James Stewart Polshek & Partners, as ''thoughtful, careful and in many ways sensitive.'' Opposed by Neighbors

Yet it concluded that a 37-story tower over the club's two-story library wing and small courtyard on East 60th Street would change the character of the landmark, with a ''loss of integrity'' and ''spatial relationships.'' And it criticized the proposed removal of a portion of the cornice on the main clubhouse, a five-story marble palazzo designed by McKim, Mead & White and completed in 1894.

Under the Metropolitan Club proposal, the club was to have sold its unused development rights to the Park Tower Realty Corporation, which would have built a 400-foot-tall building of limestone and copper. The plan had been opposed by preservationists and by the club's neighbors, including the Pierre Hotel.

The president of Park Tower Realty, George Klein, attended yesterday's meeting but would not discuss what he would do next. 'An Unfortunate Signal'

Mr. Polshek said: ''This vote could be construed as an unfortunate signal to other enlightened developers that quality architecture may not be a good investment. We sought to prove the proposition that one of the roles of exsiting landmarks was to act as a catalyst for the creation of new landmarks.''

It was for precisely that reason, however, that critics of the proposal cheered yesterday's decision.

''A landmark designation is not supposed to be an invitation to architects and developers,'' said Anthony C. Wood of the private Historic Districts Council. ''If they had approved this, it would have been a signal to other developers to come and gild the lily.''

The commission chairman, Gene A. Norman, insisted that no signal was being sent and that the commission acted case by case. In September, the panel approved a new apartment building adjoining the former Towers Nursing Home on Central Park West.

Mr. Norman said that in the Metropolitan Club plan, which affected the arrangement of clubhouse, library and courtyard, ''the new building, although of very high quality, did things to disrupt that harmony.'' Part of Historic District

The Metropolitan Club is an individual landmark and part of the Upper East Side Historic District.

Commission members voting against the proposal were Mr. Norman, Elliot Willensky, David F. M. Todd, Anthony M. Tung, David A. Garcia, Sarah Bradford Landau and Thomas J. Evans. George S. Lewis and Adolf K. Placzek, abstained.

The commission took these actions in the theater district: Broadway Theater, 1681 Broadway - Completed in 1924 as a movie theater, designed by Eugene DeRosa, owned by the Shubert Organization. Exterior and interior designations denied. The former Century Theater, 235 West 46th St. - Completed in 1925 as a nightclub, designed by Thomas Lamb. Interior designation denied. Cort Theater, 138 West 48th St. - Completed in 1912, designed by Thomas Lamb, owned by Shubert. Exterior and interior designated.

''Its facade is an exceptional example of the neoclassical style'' and its interior is ''unusually handsome.'' Embassy 1 Theatre, 1556 Broadway - Completed in 1925 for Metro-Goldwyn-Mayer, designed by Thomas Lamb. Interior designated.

''One of the few remaining intact early movie theaters in the Times Square area'' and one of ''Lamb's finest small theaters.'' 46th Street Theater, 226 West 46th St. - Completed in 1925, designed by Herbert J. Krapp, owned by the Nederlander Organization. Exterior and interior designated. ''Its facade is an exceptional design based on Renaissance and neo-classical sources'' and its interior was ''designed in the 'stadium' plan, an innovation.'' Golden Theater, 252 West 45th St. - Completed in 1927, designed by Herbert J. Krapp, owned by Shubert. Exterior and interior designated.

''It was designed as a small, 'intimate' theater seating only 800. Its facade is a fine example of the romantic 'modern Spanish' style.'' Helen Hayes Theater (formerly the Little Theater), 238 West 44th St. - Completed in 1912, designed by Ingalls & Hoffman. Exterior and interior designated. ''An elegant neo-Federal style'' exterior, with an ''unusual'' interior ''intended to suggest intimate drama.'' Mark Hellinger Theater, 237 West 51st St. -Completed in 1929 for Warner Brothers, designed by Thomas Lamb, now a legitimate house owned by Nederlander. Interior designated. ''The last surviving grand movie palace in midtown Manhattan.'' The spaces are ''an extraordinary adaptation of Baroque sources.'' Hudson Theater, 139 West 44th St. - Completed in 1904, designed by J. B. McElfatrick and Israels & Harder, owned by Harry Macklowe.

Exterior and interior designated. Among the oldest surviving Broadway theaters, its facade is a ''restrained but handsome version of Beaux-Arts classicism'' and it has ''unusually handsome auditorium and lobby spaces.'' Imperial Theater, 249 West 45th St. - Completed in 1923, designed by Herbert J. Krapp, owned by Shubert. Interior designated; exterior designation denied. The interior is an ''excellent example of the elegant Adamesque style.''

The Shubert Organization, the Nederlander Organization and Jujamcyn Theaters have joined with Save the Theatres Inc. to propose an alternative to the current landmark designation process. Rather than focusing on ornament and decoration, they have said, the panel should designate interiors largely on the basis of ''enclosure geometry'' - sightlines, stage configuration, proportion and intimacy.

Photo of the Metropolitan Club (NYT/William E. Sauro)