On the orders of KGB head Yuri Andropov, 15 agents came in May 1968 to target Czechoslovakia's intellectual elite, a major force behind attempts to reform the communist regime in the 1960s.
Milan Barta, a senior researcher at Prague's Institute for Study of Totalitarian Regimes who examined copies of documents released by Cambridge University to The Associated Press, said the operation against the Prague Spring protagonists was the first such KGB action against a Warsaw Pact ally.
He said the mission was part of the preparation for the Soviet-led military invasion that crushed the reform movement in August 1968.
The documents are copies of KGB files smuggled out of Russia in 1992 by senior KGB official Vasili Mitrokhin. Cambridge University is giving researchers access to 19 boxes containing thousands of files.
The unidentified agents were supposed to "infiltrate the reactionary circles" and to carry out unspecified special operations.
"They considered the worst enemies those who could influence public opinion through media," Barta said. "They concluded that the major supporters and driving force of the reforms are the media and people linked to them."
Two of the spies made a failed attempt to kidnap literary historian Vaclav Cerny and author Jan Prochazka.
