Uncovering Soviet intelligence; KGB, by Brian Freemantle.New York: Holt, Rinehart & Winston. 208 pp. $14.95.

Inside the Soviet Union, the KGB is an official censor. Possessing material that has been banned by this intelligence and secret police unit is a crime punishable by a term in the prison camps. The KGB has also been suspected of involvement in silencing dissent abroad ever since Georgi Markov, a Bulgarian working for the BBC in London, died after being jabbed with a poison-tipped umbrella in 1973.

In his new book, ''KGB,'' Brian Freemantle, a British journalist who has worked for the London Daily Express and the London Daily Mail, doesn't describe the work of the various directorates of the KGB in detail but does give an idea of the scope of the KGB's activities. For example, no one can be appointed to military intelligence (GRU) without KGB clearance; because the Soviet airline, Aeroflot, is considered a supporting division of the armed forces, it is within the province of a KGB directorate; another directorate controls labs in the Soviet Union and its satellites, where research and development of poisons are conducted; and yet another directorate selects assassins.

Recent Soviet intelligence activities are of particular interest because Yuri Andropov, who was KGB director during the systematic suppression of dissent under Brezhnev, became a member of the Central Committee of the Soviet Communist Party in May and is considered a possible successor to Brezhnev.

''KGB'' is not a scholarly book, nor does it go into many questions about the KGB's methods of operation. But it is a well-organized effort to get past the abundant propaganda and speculation about Soviet espionage. Freemantle draws on evidence from congressional hearings, English inquiries, press reports, and interviews to document:

* KGB control of the Soviet-bloc intelligence services in Eastern Europe.

* KGB involvement in assassination attempts like the one against Pope John Paul II in 1981 and in liquidation of dissidents in Czechoslovakia.

* Cases of long-term imprisonment of Soviet citizens for resisting KGB pressure to act as informers.

* Experimentation on prisoners who were tested to determine the effects of radiation and of poisons and who were used in various survival experiments.

Although the KGB is believed to have five times the manpower of the CIA and the intelligence services of Western Europe combined, ''KGB'' is only the third general treatment of the organization to be published in the United States since 1974, a period during which 25 books about the CIA have been published. Naturally the sources of information about the KGB are limited. There is no Freedom of Information Act in the Soviet Union, and defectors to the West have sometimes been exposed as Soviet ''disinformation'' agents. ''KGB'' needs to be viewed as a work of journalism, not scholarship, but readers interested in the subject will find that this book avoids speculation and propaganda about Soviet espionage more successfully than the other books available.

One interesting photograph in ''KGB'' is of a prison camp near the Arctic Circle where Alexander Solzhenitsyn spent several years.

Changes in the Soviet leadership under Brezhnev don't appear to be diminishing the role of the KGB in Soviet control of information about the regime. A Romanian secret service plan to murder the Romanian dissident writers Virgil Tenase and Paul Goma, and an attempt made by squirting poison from a fountain pen into a drink at a party, were publicized last month in France. These reports provide the most recent and conclusive evidence of the intelligence service methods for silencing Soviet critics.