Family members of Soviet dissident activist Anatoly Scharansky say they fear that after nearly six years in prison and labor camp, his life is in grave danger because of the effects of a hunger strike that he began in September.

Scharansky's wife Avital, who is in London on a continuing odyssey to seek her husband's release, said today that the dissident's mother was told by prison officials that he is being force-fed once every three days. She said that none of the family had heard directly from Scharansky since last January, but that authorities had now formally advised his mother that the hunger strike began Sept. 27, the Jewish holiday of Yom Kippur.

As a result of his continuing protest, Avital Scharansky said, his mother had been told that a previously promised meeting with him at the prison had been canceled. The mother, Ida Milgrom, 75, is staying in a cabin outside the prison in the provincial town of Chistopol. She has demanded to see her son, if only to appeal for him to end the hunger strike and to confirm that he is alive.

Scharansky, a diminutive 34-year-old mathematician who sought unsuccessfully to emigrate to Israel before becoming one of the mainstays of Moscow's human rights movement, was arrested on March 15, 1977 and tried on espionage charges. He was sentenced to three years in prison and 10 in a labor camp, but has been kept in prison--much of the time in solitary confinement--because of his alleged refusal to cooperate with authorities.

Avital Scharansky, who lives in Israel, has sought to keep interest in her husband's case high by traveling to meet with political leaders, diplomats and the public. Last spring she met with President Reagan, and she has seen British Prime Minister Margaret Thatcher and French President Francois Mitterrand.

Scharansky was arrested at a time when human rights in the Soviet Union was a prime focus of Western attention. But with the passing of years and the Soviets' dispersal of many prominent dissidents, the spotlight has shifted to such East-West issues as arms control.

After Scharansky began his hunger strike, his wife went to the United States and adopted what she describes as a "low profile" in hopes that it might make the Soviets more amenable to appeals. But now, she said, "I must shout with all my might again. I am not talking about the future of the human rights movement. I am talking about a man's life that is endangered unless something is done quickly."

There have been periodic reports that Scharansky's health has suffered in prison. His weight was said to be below 100 pounds at one stage, his eyesight was deteriorating and he endured severe headaches. His family attributed these problems to continuing harassment from officials attempting to force Scharansky to make a public statement of guilt.

At his trial and in the few communications he has had since with his family, Scharansky maintained an attitude of defiance despite the pressures imposed on him by authorities.

Scharansky is one of the best known of the dozens of Soviet dissidents imprisoned in a crackdown that began in late 1976 and still continues. In recent years, moreover, as detente has waned, the Soviets have also cut back sharply on the number of Soviet Jews permitted to emigrate. From a high of over 50,000 in several years during the 1970s, the total for 1982 dropped to 2,700, according to figures released this week by the Intergovernmental Committee for Migration.

Describing the efforts of Scharansky's mother to stay in contact with her son, Avital Scharansky said that Soviet officials had shown extreme callousness by refusing to follow regulations on delivery of mail and twice-yearly meetings.

When the mother met last week with the prison chief, according to Avital Scharansky, she asked details on her son. All he would say, she relayed, was, "He doesn't look like me and he doesn't feel like me."