The shadow metropolis of fandom appeared, amid thunder and hail Friday night at the Sheraton Park Hotel, and by Monday morning had vanished again, like one of the shadow worlds in the novels of Roger Zelazny.

On the hotel's bulletin board, Disclave '79 was listed merely as the Washington area's annual science fiction convention, but to the hotel's goggling guests and the meeting's costumed participants, that was obviously an inadequate description.

Long before the costume party was to begin (and for two days after it as well), the lobby was haunted by fully costumed space pilots, a medieval falconer with a live kestrel perched on his arm (the bird ignored the crowd, as it was busy tearing off shreds of a tiny white mouse in its claws), a knight in mail, Cylon warriors, and various odd uniformed and caped figures.

The guest of honor at this year's convention was writer Roger Zelazny, three-time winner of the Hugo Award, two-time winner of the Nebula award, and creator of the "World of Amber" novels that are famous in fandom.

Amber, in his books, is the only real world while all others are shadow worlds willed by the Princess of Amber.

Fandom, a world willed by the hard-core readership of science fiction, is a literary following unlike any other. The social center of fandom is the "con," the weekend convention.

The cons, according to Publisher's Weekly, which took note of the recent boom in science fiction book sales by establishing an annual science fiction issue, "are the best attended, longest running traveling sideshow in the universe. Virtually every weekend, somewhere, a local science fiction club is arranging a convention." Attendance ranges from 500 to 6,000 for the annual "Worldcon," and in the past few years of boom, these meetings have been getting larger.

This weekend at the Sheraton Park about 1,450 local fans appeared, about seven times as many as attended the first worldwide science fiction convention in 1939.

Science fiction writers have managed the ultimate literary trick in creating not only a genre of literature that is increasingly popular, but they have invented their own audience to carry around with them from hotel to hotel, weekend after weekend.

From 1926, when the modern period of science fiction began, there have been pulp magazines such as Amazing Stories, but within only a few years after the pulps began, dozens of little mimeographed magazines called "fanzines" were begun by fans to write about S-F and fandom, like historians of a non-existent civilization. Next came the conventions.

Now, says Roger Zelazny, "science fiction is unique in possessing a fandom and convention system which make for personal contacts between authors and readers . . . When an author is in a position to meet and speak with large numbers of his readers, he cannot help feeling the way ancient storytellers must have felt facing the comments and questions of a live audience."

Science fiction, because of its fandom, had become almost a participatory cultural sport, with the more avid and able fans themselves becoming writers and then visiting the conventions weekend after wekend. Among the fans-turned-writers are Isaac Asimov, Damon Knight, and Frederick Pohl.

Zelazny himself was a fan and reader for many years, and he helped edit a fanzine before going off to college to study English literature.

Zelazny, wearing white jeans and a gold sport shirt, radiates the sense of one who has excaped from the claws of ordinary life. As soon as he could support himself with his writing, he quit his government job and traveled around the United States looking for his ideal place to live. He moved to New Mexico.

For seven years after he graduated with a master's degree from Columbia University, Zelazny worked for the Social Security Administration in Cleveland, his hometown.

The job had a fancy policy title, but he says, "essentially it was writing government manuals." Did you like the job at all? "No."

He pured his evenings and weekends, beginning in 1962, into writing science fiction and sold 16 stories to the pulp magazines in his first year of work.

According to their own mythology, psychological studies have shown science fiction fans to be eccentric people-smarter than average, more stubborn and manic about details than average, and lonelier than average-essentially a collection of imaginative people who want to be part of something more exciting.

Their congealing around the hobby of science fiction was natural. As Zelazny says, the genre itself is eccentric: "The modern, realistic novel has discarded what Northrop Frye has classified as the higher modes of character. It is a democratic place, without room for heroes, rash kings, demigods and dieties. Science fiction on the other hand, retained and elaborated these modes, including mutants, aliens, robots, androids and sentient computers."

Science fiction attempts to replace the epic, he says. And so, 1,450 epic imaginations gathered to talk to one another and Roger Zelazny this weekend, to make a collective, phantasmic event, beginning with the costume party of Friday night.

There were Robin Hood figures, an executioner in hooded black, carrying a seven-foot, double-bladed axe. A woman in a sky-blue Guinevere gown led, by an inch-thick rope, a grunting, hairy, primitive man a la Edgar rice Burroughs.

One young man wore a huge, hairy wolf's head with saber teeth above a sport shirt with a shoulder patch reading, "New Orleans Police."

There was also a full complement of ordinary characters in detailed costume: Superman, Darth Vader, Ben Kenobi.

Into this crowd came a woman in street clothes, and as she walked in she asked someone lounging by the door, "Is this where the costume party is supposed to be?"

Throughout Saturday and Sunday were panels on subjects from religion and science fiction to science fiction humor.

Among the most fascinating of the events was the art show, which had first-rate artists who specialize in science fiction, including Michael Whelan, the guest artist who has been nominated for this year's Hugo award in the art category, and Ron Miller, a local artist who has been an illustrator for NASA and the Air and Space Museum.

Science fiction art, according to Whelan, is far more demanding than most people imagine, because the fans like to see detail, and believable anatomies, in creatures and settings that are almost too fantastic to draw.

Whelan, who has just finished painting the cover art for a new edition of the Mars books by Burroughs, has had anatomical chores made easier because he began studying medicine before entering art, and for several years helped prepare cadavers for classes.

The luminescent, perfectly detailed quality of his work drew crowds of fans to his corner of the art room over the weekend to study those dreams and nightmares which exist outside the ordinary waking life of art, just as science fiction books live outside the normal life of literature.

On the last day of Disclave, Roger Zelazny spoke at one of the best-attended events of the weekend. More than 500 listened, as one fan to another, he related how he works-after midnight, in a reclining chair with a typewriter hovering over his belly-and about the growth of science fiction that has brought him three movie contracts for his novels.

"I grew up on radio, and reading books and science fiction is something in my head," he said. "No one can depict an alien quite the way I would in my mind when I fill in details from the dark corners of my subconscious.

"No, I don't science fiction is something that you can put on the screen very well." He paused, and then added with an insider's grin, "But I'm willing to take Hollywood's money if they want to try."

On Sunday, as the weekend faded, going up all over the hotel's bulletin board, pillars and walls were flyers bidding fandom to come to the next outcropping of the shadow world-the cons of Philadelphia, of Chicago, and on . . . ad infinitum. CAPTION: Picture 1, Ellen Siegel, left, and Darlene Masten pose with robot-costumed Becky Neushaw; by Gerald Martineau-The Washington Post; Picture 2, Roger Zelazny; by Vanessa Barnes