Acknowledging that "Arthur Fiedler is irreplaceable," Academy Award-winning Hollywood composer and conductor John Williams announced in London yesterday that he would nevertheless succeed the legendary Fiedler as conductor of the Boston Pops Orchestra.

Hollywood is coming to Boston.

A six-month search by the parent Boston Symphony for a successor to Fiedler ended here on Wednesday with the selection of Williams, who has created and conducted the music for 60 films and won three Academy Awards, including one for his extraordinarily popular score for "Star Wars." aHe has been working in a London sound studio on the "Star Wars" sequel, "The Empire Strikes Back."

"The legend of Arthur Fiedler looms large," said Williams. "The world knows him as a beautiful-looking man who made orchestra music immensely popular. But the vastness of his reputation among professional musicians and the depth of his musical knowledge were even more impressive."

During the 50 years the imposing, white-maned Fiedler conducted the Boston Pops, it earned the right to call itself "the best-known orchestra in the world," with its summer and Christmas concert series, its tours and television appearances and its sale of 50 million records.

Williams' success in Hollywood nearly matches that record for mass acceptance of his work, although he has become no more a household name than most other successful film composers. Nominated for the Academy Award 13 times, he won Oscars for "Star Wars," "Jaws" and "Fiddler on the Roof," which he adapted and expanded from the Broadway stage.

He also produced the music for "Superman," "Dracula," "Close Encounters of the Third Kind," "The Towering Inferno," "The Poseidon Adventure" and "Goodbye, Mr. Chips," among other popular movies. For television, he composed for the Alcoa Theater, Kraft Theater and Playhouse 90, winning two Emmies.

Everything about Williams is soft, like the focus of dreamy films, in contrast to the robust charisma of Fiedler. He has a soft voice, soft blue eyes behind delicate, wire-rimmed glasses, soft pink skin on his full face and balding pate, a soft-looking, neatly trimmed brown-and-white beard, and a soft, rather pudgy body under his soft California-gray sport coat and slacks and black turtleneck.

Williams hopes to use his Hollywood success to further enlarge the role of the Boston Pops in attracting a mass public to orchestra music. "'Star Wars,' for example, has been important for music," he said. "Kids have gone out and bought the album. It was not rock, it was performed by the London Symphony, and they liked the sound."

Williams' commercial talents undoubtedly played a role in his selection.

Asked at a Boston press conference if Williams was chosen with recording contracts in mind, BSO general manager Thomas Morris said: "The basic criterion was more musical and somebody who would be the best musical leader for the Pops. If we could find that along with these other business things we would be lucky -- and we were very lucky."

"Certainly in selecting a conductor for the Pops we hoped the combination of the new conductor and the orchestra would be attractive for recording and for television," said Morris. "And certainly that came up positive in the case of John Williams. I don't look at it as a carrot but as an opportunity with him."

In London, Williams himself talked with evident sensitivity of forgotten American composers he would like to revive with the Boston Pops, new young composers and soloists he would like to showcase, and electronic sound techniques he would like to try. But he spoke almost reverently about Fiedler's vast and successful popular repertoire, much of which Williams intends to preserve. "Changes in repertoire," he said, "can only be slow and gradual."

"This decision is not a career decision, nor a financial decision, although I am amply satisfied with my new arrangement," said Williams, a widower after 18 years of marriage. "I see this as primarily a musical decision, a chance to make music on a level I haven't had."

Although he never met Fiedler, he talked on the telephone with him several times, including once last year when Fiedler called to ask Williams to compose "five minutes of brilliant march music" for Fiedler's 50th anniversary celebration.

"Unfortunately, I couldn't do it because I was working on a film," Williams recalled. "Perhaps now I will write a march, dedicate it to him and play it with the Boston Pops this year."

Williams' appointment officially closes the era of "The Maestro," as Fiedler was called.

Fielder, who died in July, was a Boston local -- a mustachioed figure with a frenetic shock of white hair who conducted the Pops from his berth beneath a bust of Beethoven at center stage in Boston Symphony Hall for nearly half a century.

Fiedler organized a series of free outdoor concerts on the Boston Esplanade at the foot of Beacon Hill on the east bank of the Charles River. Under his direction, the orchestra sold 50 million records and became an institution on public television with "Evening at the Pops." Fiedler drew the largest crowd in history to an orchestral performance for his Bicentennial July 4 concert, in which half a million people streamed to the banks of the Charles River for the performance capped by Tchaikovsky's "1812 Overture" replete with fireworks, cannons and church bells ringing across the city.

He ended his career in a series of concerts making his 50th anniversary with the orchestra last year.

But even before Fiedler's death, a committee was formed to begin looking for his successor -- someone who could retain the enthusiasm that has brought the Pops a lucrative recording and public appearance schedule. Morris said the Pops has contributed a "substantial" percentage of the symphony's income.

Morris said Williams will arrive in Boston a week from Monday and will rehearse the orchestra for a concert he will conduct at New York's Carnegie Hall Jan. 22. Williams has written a new tune for the occasion called "Cowboy's Overture," which Morris said has been described "as a real corker -- not by him, as a matter of fact." The first performance in Boston is April 20 at a fund-raising benefit for the orchestra.

Williams has signed a two-year contract obligating him to appear in Boston for the entire Pops season of May, June and July. He will conduct a majority of the concerts at the symphony and on the Esplanade, as did Fiedler.

Morris said the tradition and format of the Pops would remain essentially the same over the short run, which is six concerts a week and includes a varied repertoire from light classical to more popular music.

"I suspect there will be some subtle changes in the repertoire in terms of different kinds of arrangements," he said, adding that Williams would like to encourage composers to do some writing specifically for the Pops.

Williams said he also hopes "to give film music more prestige" by presenting the best of it with the Boston Pops. "Of all the music written in the 19th century," he said, "we play only about one-half of one percent of it because the great volume was not that distinguished. It's the same with film music. I believe the best of it is very good, and I want to present what I think is the best."

Williams said he intends to continue producing music for films at the rate of about one each year. He will score Stephen Spielberg's next film, "Raiders," in early 1981, and the third film in the "Star Wars" trilogy in early 1982.

Educated in music at UCLA and the Juilliard School of Music, Williams thinks of himself as a very serious as well as a popular musician who has written symphonic works in addition to his prodigious Hollywood output."The art of music has been a lifelong passion," he said softly. "I'm 47 years old, and I've been studying music for 40 of those years."