In an extraordinary letter smuggled recently to the West, exiled Soviet human-rights activist Andrei Sakharov has written that he was drugged, strapped down, force-fed and mentally tormented by KGB agents and state doctors seeking to end the hunger strike he launched in 1984 on behalf of his ailing wife.

"What happened to me that summer is strikingly reminiscent of Orwell's 1984," Sakharov wrote. "In the novel and in real life, the torturers sought to make a man betray the woman he loves."

Sakharov described repeated physical abuse in a hospital in the provincial city of Gorki, including doctors' use of "a tight clamp" on his nose to force him to open his mouth to accept food. "Orderlies would throw me onto the bed, tie my hands and feet and then hold my shoulders down while the needle was inserted into a vein . . . . Sometimes my jaws were pried open by a lever," he wrote.

After being forcibly injected with a mysterious substance Sakharov passed out, and when he awoke he suffered from bizarre optical illusions. "I found out later that this sort of optical illusion is symptomatic of a spasm in a cerebral hemorrhage or stroke," he wrote, and later he suffered from other symptoms associated with stroke. This "spasm or stroke I suffered . . . was not an accident; it was the direct result of the medical measures taken in my case on orders from the KGB."

Sakharov also recounted the diagnosis of doctors in Gorki that he suffered from Parkinson's disease. They told him that if he persisted in his hunger strike "there would be a rapid and catastrophic development" of the disease, leaving him a helpless invalid.

The letter, to be published in today's London Observer and this week's editions of U.S. News & World Report, is the first direct account by the Nobel Peace Prize laureate describing the confrontation over his effort to gain permission for Yelena Bonner, his wife, to seek medical treatment in the West.

Bonner was allowed out of the Soviet Union last November and is recuperating from heart-bypass surgery she underwent in Boston last month. Moscow's decision was widely portrayed as a humanitarian gesture by the new Soviet leader, Mikhail Gorbachev, at the time of last autumn's Geneva summit with President Reagan.

Sakharov's letter, dated Oct. 15, 1984, makes clear the decision was preceded by a pitched battle between Sakharov and the secret police that was set off when the KGB kidnaped him and confined him incommunicado for four months in a guarded hospital in Gorki, 230 miles east of Moscow, where he has been held in exile for six years.

Sakharov recounts how on May 7, 1984, five days after he began a hunger strike, secret police "disguised in doctors' white coats" seized him as he was accompanying his wife to a police interrogation, and took him to the hospital.

"From May 11 to May 27, I was subjected to the excruciating and degrading process of force-feeding," he wrote. "The doctors . . . kept changing the method of force-feeding . . . . to maximize my distress in order to make me give up . . . .

Sakharov wrote the letter to Anatoli Alexandrov, president of the Soviet Academy of Sciences, to enlist his aid on behalf of Bonner. Sakharov has remained a member of the prestigious academy despite his forced exile to Gorki and other official measures against him, though in this letter he threatened to resign his membership.

The physicist's detailed rendition of how the KGB handled his challenge to the state contradicts the official Soviet version of the conditions of his exile. Soviet media portray Sakharov, once the most esteemed scientist in the country, as living in pampered conditions, pursuing his research.

Publication of the letter comes on the eve of an important Communist Party congress in Moscow, when Gorbachev is expected to push for reforms of the creaking economy and call for greater political discipline.

Gorbachev recently told the French communist newspaper L'Humanite that Sakharov will not be allowed to emigrate from the Soviet Union because he knows nuclear secrets from his work as a weapons-builder in the 1950s and '60s.

Efrem Yankelevich, Sakharov's son-in-law, said yesterday that the family received this letter "and other documents" in packages from a European country. He said he and his wife, Tatiana, who is Bonner's daughter by a previous marriage, are "convinced beyond doubt" that the letters and documents are authentic. He said publication was not timed to the party congress or any other specific event.

Yankelevich said the Observer had paid the family $42,000 for world rights to the letter and other documents to be published later. The money will be used to pursue the causes Sakharov supports and to further efforts to win his release from Gorki, Yankelevich said.

U.S. News & World Report released the text of the letter yesterday. It bought rights to the document from the Observer.

Yankelevich said Bonner talked by telephone with Sakharov on Friday, but did not tell him of the publication of the letters. "He does not know of this," Yankelevich said. "But he gave me a power to decide this eight years ago, and as he almost always automatically publishes his views, I am certain he approves of this."

Bonner was given a 90-day visa to come to the West for medical treatment after she promised to make no statements to the news media. She has asked the Soviet embassy here to extend the visa to allow a fuller recuperation, but the embassy has not yet acted on the request. Some family friends have expressed the fear that Bonner might not be allowed to return to Gorki but Yankelevich has said he cannot imagine that the Soviet authorities would willfully separate spouses in such a manner.