TALLINN, U.S.S.R. -- Mart Niklus was sentenced to 10 years' imprisonment in 1980 for "anti-Soviet agitation." His crime was publicizing the facts about the Soviet Union's forcible annexation of his native Estonia in 1940.

When he was released last July, Niklus was amazed to find in Estonia's Communist-run mass media details of the secret pact between Joseph Stalin and Adolf Hitler that led to the incorporation of his Baltic country into the Soviet Union. Views that had earned a lone dissident eight years in a "strict regime" labor camp in the Urals were now being openly expressed by tens of thousands of ordinary Estonians.

To add to his sense that he was in the midst of a surrealistic experience, the 57-year-old human rights activist was selected to be an official delegate to the founding congress of the Estonian Popular Front earlier this month. He insisted on wearing his prison clothes to the gathering, taking his place among doctors, factory managers and Communist Party members, all in respectable business suits.

"It would have been impossible to imagine all this 10 years ago," said Niklus, still somewhat dazed by the degree to which his life -- and, by extension, Estonia -- has changed over the last few months.

A few months ago, Niklus was a stubbornly independent inmate of prison camp VS-389/36-1 near the Soviet city of Perm, center of the modern-day Gulag system. Considered a "dangerous recidivist" by the camp authorities -- he had served three previous prison terms for his human rights activities -- he was kept in strict isolation from other prisoners.

Camp routine consisted of getting up at 6 a.m., having a small piece of bread and mug of watery tea for breakfast and sewing 522 electric cords onto 522 electric irons every day. Lunch was usually vegetable soup and oatmeal, dinner fish soup and a small piece of sausage. During the afternoon, he was allowed a 45-minute walk in a 30-foot-long cage.

"Animals in a zoo are kept in better conditions than people in our prisons," said Niklus, a former zoologist and English teacher.

The one concession made by the authorities to the prisoners' thirst for information about the outside world was to allow them to read Soviet newspapers. Niklus read Estonian press reports about demonstrations back home demanding his release. On June 30, still sitting in prison, he read an interview with a senior official announcing that he had already been released.

"It was a brilliant example of Soviet bureaucracy," Niklus quipped. His reaction was to declare an immediate hunger strike. Finally, on July 8, he was freed. He set off by train for Moscow and his home town of Tartu in Estonia.

Hoping that someone would show up at the train station to help him carry his luggage, Niklus had telegraphed his mother with news of his imminent arrival. But he was unprepared for the sight of hundreds of people on the platform, offering him flowers and waving the newly legalized flag of the prewar independent Estonia.

"It was the most fantastic moment of my life. They didn't simply carry my bags -- they carried me, on their shoulders. Everyone wanted to touch me," Niklus recalled.

Now that he has had a few weeks to absorb the changes that have taken place in Estonia, the former dissident is at once enthusiastic and skeptical. He concedes that Soviet President Mikhail Gorbachev has introduced some positive reforms and that Estonia's new leaders are better educated and more sophisticated than their predecessors. But he worries that it could all be easily reversed.

"We have gained many things, but we are still an occupied country. Until Estonia's independence is restored, nothing really serious can be expected. I have heard so many empty promises in my lifetime that I have stopped believing in a radiant future."