A 50th birthday party for Radio City Music Hall

Having survived the kind of threat that is demolishing the Morosco and Helen Hayes Theaters, the landmark Radio City Music Hall is observing its 50th anniversary by celebrating itself and its entertainment tradition.

''Encore,'' as the celebration is called, offers Music Hall fans a gorgeous song-and-dance extravaganza, a 90-minute spectacular that blends past highlights into a pop potpourri of solos and ensemble numbers. The spirit of the occasion is perhaps best summed up in the semi-finale, the Dietz-Schwartz ''That's Entertainment,'' in which the high-kicking Rockettes and their numerous fellow performers make any further definition of ''Encore'' superfluous.

Following Stan Lebowski and Fred Tobias's musical salute to ''the Showplace of the Nation,'' the talking-picture image of Ginny Hounsell appears on the giant multisectioned screen. A Rockette designated as the Music Hall's Golden Jubilee Ambassador-to-the-World, Miss Hounsell conducts a zip-along guided tour through an animated collage of photos and film clips from Music Hall history. (She might well have been spared the gurgling synthesizer accompaniment.)

The ambassador's introduction is followed by ''The Glory of Easter,'' an ornately florid blend of Albert Hay Malotte (''The Lord's Prayer'') and Anton Rubinstein (''Kamenoi Ostrow'') in a Music Hall special first presented in 1933. The second seasonal interlude is a choreographed cherry blossom number featuring a display of the Doncho Curtain, presented to New York City in 1964 by the city of Tokyo. It is said to be the largest hand-woven silk theatrical backdrop ever created. Whether as picturesque scenery or exquisite public relations gesture, the effect is breathtaking.

''Encore'' also returns to its past for danced versions of Gershwin's ''Rhapsody in Blue'' and Ravel's ''Bolero,'' strikingly choreographed for the occasion by Geoffrey Holder, assisted by Linda Lemac. In the Kander-Ebb ''Showstoppers,'' dance director Violet Holmes puts her girls through a bit of simulated rehearsal, a backstage look at the working life of a Rockette.

''Fifty Years of American Popular Music,'' one of the liveliest medleys on the program, offers a 17-minute salute to composers ranging from Berlin and Bacharach to the Beatles. Here and elsewhere, the show draws on the likes of Cahn, Rodgers and Hart (and Hammerstein), Porter, Styne, Arlen, Allen, Warren, Harburg, Mercer, Ellington, and many others. For a touch of the classics, Joseph Klein leads the busy and movable Music Hall orchestra in a smattering of the over-the-years overtures, including Verdi, Rossini, and Rimsky-Korsakov.

''Encore'' was produced and directed by Robert F. Jani, assisted by Tom Bahler (musical direction), Charles Lisanby (scenery), and Ken Billington (lighting). Besides those mentioned, the choreographers are Adam Grammis, Shozo Nakano, and Frank Wagner. Michael Casey and Bob Mackie designed the original costumes, with those for ''The Glory of Easter'' and ''Rhapsody in Blue'' based on designs by Frank Spencer and those for ''Bolero'' based on designs by Vincente Minnelli.