Soviet book-lovers took over the first International Book Fair here today, gazing, thumbing, reading - and sometimes making off with - volumes of foreign publishers they had never seen before and may never see again.

Western book publishers reported that large-format art books, especially those dealing with modern painters and sculptors, were among the most popular, with sports ad children's books also high on the list. The throngs were so great that many publishers had to cordon off their booths, letting a few dozen persons at a time to avoid a crush and to keep a better eye on their stocks.

Some of the sales representatives seemed dazed by the eagerness of the fair goers, who went from booth to booth filling their pockets with catalogs and brochures and talking with friends and strangers about the best places to browse.

Several publishers said books had been stolen from their display racks in the first two days of the fair, which is to end next Wednesday. About 10 books were missing from a Times-Mirror Co. booth, and Penguin Books reported the loss of several dozen. But perhaps the leading loser was a small husband-and-wife publishing house Mich. The owners, Carl and Ellendea Proffer, said afout 30 of their 75 titles disappeared the morning of the first day and 10 more were taken today.The Proffers, who are Slavic scholars, publish Slavic and Soviet writers in transaltion. The loss of the 30 books was discovered just after official Soviet book-buyers and other Soviet specialists had crowded their booth. As a result, the Proffers cordoned off their area and allowed only a dozen or so persons in at a time.

Ellendea Proffer said customs officials appeared at mid-afternoon today and impounded all copies of a catelog the Proffers were going to give to officials of "Vaap," the Soviet copyright agency. The catalog apparently was seized because it contains a description of a book about the works of exiled Soviet writer Alexander Solzhenitsyn.

Dissident Russian writers could not be found on the shelves of the exhibit, but they were there in the flesh, walking around the two vast halls of the book fair inspecting books and mingling with the crowds. Roy Medvedev, Vladimir Voinovitch and Lev Kopelev have been here. Their books are published in the West but are banned here. The catalog of one British Publisher was seized yesterday because it referred to "The Ivaniad," a book by Voinovitch. One of these writers said today he will return each day "to excercise the rights of a Soviet Citizen." Sources said the writers, well-known to Soviet security police, were moving freely from exhibit to exhibit.

Several Americans said they had been told that Mikhail Sholokov, 72, author of the modern Russian classic "And Quiet Flows the Don," is eexpected to tour the fair.

Most Americans ascribed the theft of books to a genuine love of reading by the Russians, who are eager for variety and cherish unexpurgated foreign editions, even though some of the titles on display here are simply Western editions of works available in the Soviet Union. Many works by such contemporary Americans as Edward Albee, Tennessee Williams, William Styron, John Updike and Ernest Hemingway are officially approved here.

But one knowledgeable Russian offered another reason - a flourishing black market in Western books.

"A book by Vladimir Nabokov that costs $8 in America can bring 50 rubles (about $65) here, he said. A wellinformed Westerner said American books are bought for large sums even by people who can read only Russian. "It is the idea of having something special from the West," he said, and they can always resell it for a high price."

An American publisher told how a Communist Party member in a group of official visitors demanded a copy of a book.

"He said he was an important official," the American said. "I told him this was going to be democratic - no special privileges for anyone."

The Americans couldn't agree on whether the pilferage was greater or less than at other fairs, but none seemed concerned about it. Most were looking for ways to distribute their books anyway, to avoid paying freight charges home.

The American publishers seemed to enjoy seeing so many ordinary folk swarming to look at their wares. "They really enjoy reading," said Harry Lerner, president of Lerner Publications of Minneapolis. He said a father sat for nearly an hour translating children's books from the Lerner display for his young son. "We are getting quite friendly," Lerner said.

The publishers were low-key and noncommittal about the eight books impounded the first day by customs officials. "We're trying to work it out and so are they," said one.

The fair is laid out so that almost all the American and British publishers are in Pavillion No. 2, with a modest sign out front. Most of the Soviet publishing houses, as well as publishers of the Eastern European countries and such nations as Japan, Bangladesh and Vietnam, are housed in Pavilion No. 1, which has an enormous sign outside as well as several flags. Some fairgoers have expressed anger and frustration upon discovering that there is just one American publisher in Pavilion No. 1, and one snapped: "Where are the U.S. people? I thought this was supposed to be international." A Western reporter directed him across the courtyard to the other pavilion.