Why `Phantom' is an immediate hit. Lyrical Lloyd Webber score is a key contribution in this romantic melodrama

The Phantom of the Opera By Andrew Lloyd Webber (music) and Charles Hart (lyrics); additional lyrics by Richard Stilgoe. Book by Stilgoe and Lloyd Webber. Directed by Harold Prince. Choreography Gillian Lynne. Starring Michael Crawford, Sarah Brightman. LUSH melodies ardently performed. Lavish spectacle and stunning effects. Overflowing sentiment and luxuriant fantasy. In other words, ``The Phantom of the Opera'' has arrived at the Majestic Theatre with its spectral thrills and coloratura trills intact.

Britain's Andrew Lloyd Webber has triumphed again, this time with a musical extravaganza that may be his most accomplished as well as (according to present indications) his most popular to date.

The mysterious tone of the evening is established immediately in visual terms. Designer Maria Bjornson shrouds the onstage antiquities and the surround of suggestive gilt statuary in huge dustcloths, which drop away to reveal the musty auction that forms a 1905 prologue to the melodrama. Among the items is a papier-m^ach'e musical box in the shape of a barrel organ with a monkey in Persian robes playing the cymbals. Discovered in the vaults of the opera house, ``the item is still in working order,'' the auctioneer announces.

The same might well be said for the reusable plot borrowed by Mr. Lloyd Webber and Richard Stilgoe from Gaston Leroux's 1911 French thriller set in ``La Belle 'Epoque.'' The adapters have turned the sturdy romance into a musical spectacular that is at the same time a vehicle for stars Michael Crawford and Sarah Brightman and a dazzling piece of collaborative stagecraft. Thanks particularly to Mr. Crawford's heartfelt Phantom, the tale of a monster who falls in love with his beautiful prot'eg'e rises above the level of mere sentimental escapism to tug at the heartstrings.

``The Phantom of the Opera'' is melodrama in almost the original meaning of the word, defined by Webster as ``a sensational or romantic stage play with interspersed songs and an orchestral accompaniment.'' True, the Lloyd Webber songs are more integral than incidental. But sensationalism and romance abound. In his brilliantly stylish staging, director Harold Prince has seen to it that the three characters of the central romantic triangle - the Phantom (Crawford), Christine Daa'e (Miss Brightman), and Raoul, Vicomte de Chagny (Steve Barton) - never become lost in the bustle and turmoil of backstage life at the opera house.

Melody is the golden constant of the melodrama. Throughout most of his career, Lloyd Webber has provided his musicals with such immediately identifiable signature tunes as the title song from ``Jesus Christ Superstar''; ``Don't Cry for Me, Argentina,'' from ``Evita''; and ``Memories,'' from ``Cats.'' In the present instance, ``Angel of Music,'' ``The Music of the Night,'' ``All I Ask of You,'' and ``Wishing You Were Somehow Here Again'' are merely outstanding creations in a score that ranges from heartfelt love aria to recitative, operatic parody, patter song, and the ominous organ peals of the Phantom theme.

Whether he is sardonically threatening the opera's blockheaded new impresarios or tenderly wooing Christine, Crawford creates a creature both frightening and vulnerable, a grotesque whose mesmerizing power springs alike from the mystery of his origins and his role as the uncrowned king of the subterranean realm beneath the opera house. Crawford soars vocally with the emotion of the music and of Charles Hart's lyrics.

Miss Brightman, who is making her Broadway debut, responds to the Phantom's hypnotic ardor as a Christine whose vocal gifts equal her personal charm and beauty. The young woman's transformation as an artist is accompanied by conflicting emotions as she yields, resists, and ultimately performs the impassioned kiss that frees her and leads to the Phantom's sacrificial act.

As Christine's aristocratic lover, Steve Barton gives the kind of strong baritone performance that could restore operetta leading men to fashion. The world of period operatic make-believe is given its lively due by a cast that includes Cris Groenedaal and Nicholas Wyman as the foolish impresarios, Judy Kaye and David Romano as a pair of caricatured opera stars, Leila Martin as the strict ballet mistress, and Elisa Heinsohn as her daughter, Christine's loyal friend. (Patti Cohenour performs the role of Christine on Thursday evenings and at Saturday matinees.)

The creators of ``The Phantom of the Opera'' have been prodigal with spectacles. Playgoers down front at the Majestic are almost part of the action in the great chandelier descent.

But the wonders of the opera house - from its dreamlike, candle-lit subterranean lake to its shadowy rooftop - are all part of the effects, which can be both special and breathtaking as lighted by Andrew Bridge.

In addition to Degas-like balletic interludes, the ``Masquerade'' of the New Year's ball presents choreographer Gillian Lynne and her dancers an irresistible opportunity for swirling motion in some of Miss Bjornson's most gorgeous costumes. David Caddick's musical direction responds equally to the big effects and the delicate lyricism of the score.