The Soviet Union today granted a six-month exit visa to one of its few remaining prominent dissidents, former Red Army Gen. Oyotr G. Grigorenko.

The action continues a major new effort - ridding the country of dissenters - in its campaign to break up and silence the small group of human rights activists and dissidents who have accused the Kremlin of suppressing civil liberties in the Soviet Union.

The departure of Grigorenko, 70, whose frequent detentions have made him a symbolic figure in the dissident movement, will leave Andrei Sakharov, the physicist who won the Nobel Peace Prize, as the sole prominent leader of Moscow's dwindling group of human rights activists.

For more than ten years, Grigorenko has been a voluble critic Kremlin human rights policies and organized dozens of news conferences to make his protests know to Western reporters.

Grigorenko said tonight that he received permission from Soviet authorities earlier today to travel for six months to the United States with his wife and stepson. He said he intends to return here when his visa expires. Grigorenko said he plans to visit another stepson already living in the United States and also to undergo an operation of a prostate condition.

In recent weeks, about 10 dissidents and their families have been granted exit visas at their own request or frightened into applying to leave the Soviet Union permanently to avoid facing certain criminal charges. This policy of deportation is viewed here as a slightly modified and less controversial approach by the Kremlin to its dissenters while the European security conference is disscussing human rights in Belgrade.

There, the Soviet Union, the United States, and 33 other nations are discussing their compliance with the human rights provisions of the 1975 Helsinki agreement on European security and cooperation.

In the spring and summer, well before the conference began, government secret police arrested more than a dozen dissidents here and in other cities around the country. The wave of jailings brought a storm of protest in the West. Now, the Kremlin has concentrated on granting exit visas, frequently to men and women who do not want to leave this country under any circumstances short of the threat of sure imprisonment.

Grigorenko, a magnetic figure at various public demonstrations and trials he has attended, confined his account of the visas to three short sentences.

A major general of World War II with the Red Army, he received many decorations and after the war made chief of the cybernetics department at the Frunze Military Academy, a prestigious institute in Moscow. In 1960, however, he began criticizing then Kremlin leader Nikita S. Khrushchev for allegedly returning to Stalinera repression. Grigorenko was broken to private and sent to the Chinese frontier.

The KGB state secret police arrested him in 1964, and he spent 14 months in the Serbsky Psychiatric Institute for what was officially described as "temporary insanity." In 1968, he and dozens of others, protested the Soviet invasion of Czechoslovakia and supported the liberalization of Prague regime that had triggered the intervention. He was arrested again and again sent to psychiatric hospitals for observation.

Diagnosed as having suffered "paranoid development of the personality with the presence of reformist ideas," he was incarcerated in the Serbsky Institute and in the provincial city of Kazan until 1974. An account he wrote of his treatment in a Soviet psychiatric ward was smuggled out to the West in 1970. It presented a searing picture of torture, deprivation and abuse of political prisoners by staff members of the ward.

Grigorenko, a tall bald man, was released in 1974. His wife, Zinaida, campaigned hard for his release. She and her son, Oleg, will accompany Grigorenko to the United States. Another son, Andrei, lives in Long Island City, N.Y. He emigrated in 1975.

Grigorenko is a friend of Andrei Sakharov, the dissident physicist, and he frequently invited Western reporters to his apartment to introduce them to other human rights activists. For many years, he has interested himself especially in the plight of the Crimean Tatars, who were forcibly deported from their homeland by Stalin in the 1930s and 1940s.