More than a third of Russians say the Soviet Union was correct to intervene in Czechoslovakia in 1968 and nearly half of the population says it knows nothing about the invasion at all, according to new polling data obtained by the Guardian before its release on the 50th anniversary of the crushing of the Prague spring.

The polling data reflects the resurgence of “Brezhnev-era propaganda, stereotypes of the Soviet period,” said Lev Gudkov of Russia’s Levada Center, which will release the results on Monday.

More than a fifth of Russians blamed a “subversive action by western countries” to split the communist bloc for a Czechoslovak programme of liberalisation that ended in a Soviet-led invasion of the communist country.

The Warsaw Pact intervention was seen as a turning point for the Soviet legacy in Europe, but its anniversary on Monday will largely pass unnoticed in Russia, where politicians and television stations have tended to stay quiet on the topic.

“Generally speaking, the authorities don’t want to pay any attention to the anniversary,” said Andrei Kolesnikov of the Carnegie Moscow Center.

Nostalgia has grown for the Soviet Union in Russia, where the communist legacy is largely associated with the victory over Nazi Germany in the second world war and its superpower status. Observers, however, say that less vaunted moments in the Soviet past are being forgotten or reinterpreted through the prism of conspiracy theory.

On the night of 20 August 1968, Soviet and other Warsaw Pact troops entered Czechoslovakia in tanks to halt a campaign of liberal reforms billed as “socialism with a human face”. The reformist Communist party leader Alexander Dubček was ousted. The intervention, accompanied by photographs of Soviet tanks in Prague, led to western condemnation and a split with some western Communist parties.

The invasion also left more than 80 dead, many from gunshot wounds, according to internal Czechoslovak reports released 20 years later, in the twilight of communism.

“These events are being forced out of the public memory,” Gudkov said.

The poll showed that an about 36% of Russians thought the Soviet Union certainly or was likely to have “acted correctly”in sending troops into Czechoslovakia. Another 45% had difficult answering whether the Soviet Union acted correctly or not – an increase from 34% in 2003.

Just 10% of 18 to 35-year-olds said they knew about the Prague spring, Gudkov said. “Young people don’t know and don’t want to know about what happened,” he said.

The new polling results reflected popular reactions to Russian geopolitical realities, such as the country’s annexation of Crimea in 2014, he said. “I think this is the manifestation of the mass amorality of a great power, which has become the basis of the Russian imperialist revival under Putin and the support of the annexation of Crimea.”

About a quarter of Russians said they had heard about the eight protesters who held a demonstration in Red Square in 1968 to protest against the invasion, holding signs that read “we are losing our best friends” and “for our freedom and yours”. The protesters later spent years in prison camps or locked away in psychiatric wards.

“The state’s interest is to hide the real meaning of the historical events of this kind. In their interpretation Russian history is the history of statesmen and military men, not citizens,” Kolesnikov said.

The events have also been tainted by conspiracy theory, he said. A television documentary in 2015 called Warsaw Pact: Declassified Pages was so aggressive in justifying the intervention that it provoked a démarche from the Czech foreign ministry. Unverified reports about secret arms caches were repeated in the documentary.

In the poll, 21% of respondents blamed a Western plot and 23% blamed a coup attempt by anti-Soviet leaders in Czechoslovakia for the events of 1968. An estimated 18% of Russians called it a “rebellion against a regime installed by the Soviet Union” – down from 31% in 2008.