The Soviet Union is defending the invasion it led into Czechoslovakia 20 years ago to crush the "Prague Spring" reforms.

But dissident Andrei Sakharov and other Soviet activists contend the Czech reform effort was "the first perestroika," referring to Soviet leader Mikhail S. Gorbachev's campaign for restructuring and greater openness.They worry that official Soviet refusal to re-evaluate the invasion reflects a limited commitment to reform.

"How can (the invasion) be defended?" Sakharov, 67, the most prominent Soviet dissident, said in a recent interview. "The invasion was a shame on our country and all who sanctioned that action."

The Czech movement for "socialism with a human face" culminated in the spring of 1968 under Communist party chief Alexander Dubcek.

On Aug. 20-21, 1968, the Soviet Union led 2,000 tanks and 600,000 troops from Warsaw Pact countries into Czechoslovakia to crush the movement.

"Now, when we think about the events of those days in August we have to consider that it was the first expression of glasnost and the first perestroika process to appear in the socialist camp," Sakharov said.

Larisa Bogoraz, a Soviet activist who spent four years in Siberian exile for protesting the invasion, said she saw little difference between the Czech and Soviet reforms. "The Prague Spring was a form of perestroika," she said.

Soviet Foreign Ministry spokesman Gennady I. Gerasimov was asked to compare the events leading up to the Prague invasion and Gorbachev's campaign for perestroika, or rebuilding.

"The difference is 20 years," he replied. But he defended the invasion as necessary to prevent "anti-socialist forces" from pushing Czechoslovakia off the communist path it had followed the 20 previous years.

Tass, the official Soviet news agency, on Friday also staunchly defended the invasion and accused Dubcek supporters in 1968 of trying to "tear Czechoslovakia away from the socialist community."

Tass concluded that "the friends and allies of socialist Czechoslovakia could not remain indifferent."

Sakharov, who says the Prague Spring helped him decide his own political course, was incredulous that the Kremlin apparently was refusing to re-evaluate the invasion.

"It is very significant that the Soviet leadership is defending this," he said. "It means we are now living through a very difficult period in perestroika and we find ourselves in danger."

In 1986, Gorbachev released Sakharov from internal exile ordered seven years earlier as punishment for his denunciation of Soviet intervention in Afghanistan.

The Nobel Peace Prize laureate is visibly fatigued and less politically active now, but he occasionally intercedes on behalf of dissidents or causes to which he is committed.

"All of us who sympathized with the reforms of our brothers in Czechoslovakia were very shaken by the invasion," he said.

"It was most important that there could be found some people in our country who were willing to uphold its dignity," he said, referring to Bogoraz and seven others who staged protests four days after the invasion.

On Aug. 25, 1968, Bogoraz and her companions gathered on Red Square and unfurled banners reading "Long Live the Independence of Czechoslovakia" and "Shame on the Occupiers."

Soviet soldiers immediately arrested the demonstrators and seized their signs, Bogoraz said, and she was exiled to Siberia. Six of her fellow protesters also were sentenced to labor camps or exile, or were committed to psychiatric hospitals.

Sakharov and Bogoraz both recalled that in 1968, authorities convened large public gatherings at Soviet factories and institutes and speakers applauded the military action that installed in Prague one of the Soviet bloc's most conservative governments.

Bogoraz and fellow activist Sergei Kovalev said they look to perestroika to further loosen the contraints on free expression in the Soviet Union.

She recalled a recent encounter with small groups gathered at Pushkin Square to discuss thorny political subjects such as Soviet involvement in Afghanistan.

"It was amazing to hear these subjects discussed openly," Bogoraz said. "There is now tremendous pressure for freedom, and maybe someday we really will see socialism with a human face."