The widening crackdown against dissenters took a new and important turn tonight with the first official acknowledgement by Czechoslovak authorities that four leaders of the movement for greater individual freedom had been arrested.

The four men - a well-known playwright, a journalist and two theater directors - were all charged with "serious crimes against the basic principles of the republic." Dissident sources had told Western reporters of some of the arrests last week.

The official acknowledgement by the national news agency that these men had been arrested suggests that the 11-day-old crackdown against signers and supporters of a new human-rights manifesto may be leading the government to its first round of major political trials since the purge that followed the fall of the liberal Communist government of Alexander Dubcek after a Soviet-led invasion of this country in 1968.

Three of the arrested men are signers of "charter 77," which was slipped out of Czechoslovakia and published in several Western newspapers earlier this month.

The charter, signed by more than 280 persons, says Czechoslovak citizens are deprived of individual freedoms assured the nation's laws and human rights guaranteed by the 1975 Helsinki agreement signed by 35 European nations, including Czechoslovakia.

At the same time the government moved against Charter 77 signers, two other letters by prominent human-rights campaigners were made available to Western reporters.

Both contained emotional appeals to Western Communist parties and other socialist forces to end what the writers view as a "witch-hunt" by Czechoslovak authorities against the human-rights movement.

Former Communist Party Secretary Zdenek Mlynar, a persistent supporter of a more liberal form of socialism, wrote that democratic and socialist forces in Europe must decide whether they will allow "supporters of internationally accepted pacts on human and civil rights in Czechoslovakia to be brutally suppressed for the second time in a decade."

Pavel Kobout, a Czechoslovak playwright who has a play running on Broadway that he has never seen produced, said the new "witch-hunt" here undermines "not only the future of socialism but also efforts in favor of a real detente in Europe."

Among the four men whose arrests have been officially announced is playwright Vaclav Havel, a spokesman for Charter 77 who had not been seen since Friday night. Havel was a strong supporter of Dubcek and a critic of the more hardline Communists governments that followed Dubcek's downfall.

Journalist Jiri Lederer and writer-director Frantisek Pavlicek were also active in the more liberal brand of Communism here nine years ago.

The fourth man whose arrest was confirmed, Ota Ornest, was director of three Prague theaters in the 1968 "Prague spring" era. He is the only one of the four who is not believed to have signed the manifesto, which suggests to some observers here that the government is not limiting its crackdown to signatories.

Until tonight's reports by the Czechoslovak news agency, news that about a dozen dissidents had been detained for police questioning for varying periods came only through conversations with reporters here or via telephone or through relatives. The telephones of many dissidents have now been removed.

In recent days, however, the government has made its attacks on the dissident increasingly public, with slashing editorials in the government and party newspapers and statements of condemnation against the dissidents from thousands of workers.

The dissidents generally are portrayed as servants of the West, well-paid and privileged intellectuals or former officials who have either turned their back on the Czechoslova brand of socialism or whose writings do not reflect the country's.

The charges against the four men are very serious and come under a special section of the penal code that deals with subvesion and antistate activities. They carry penalties that range from 2 to 15 years and even, in some cases death.

The only name actually mentioned in the official report was Ornest. The others were alluded to by their initials, but sources said there was no doubt as to whom the report referred.

The report charges the four with "prolonged contacts with foreign hostile forces an demigre centers." It adds that they passed on material harmful to Czechoslovak interests that was misused in foreign media and then secretly smuggled back into Czechoslovakia.

The report says that for these activities "the arrested individuals even used employees of some diplomatic missions of capitalist state."

The increasingly harsh conflict between the dissidents and the government poses a dilemma for the government of Gustav Husak.

While the government seems willing to take stern measures, despite some embarrassing news accounts in the Western, the over government activities are having the effect of making more people here aware of the human-rights campaign.

Czechoslovak sources say that when it was first published in the west, virtually one here except intellectuals knew about Charter 77. Then, as attacks increased, politically sensitive people became aware of the charter's contents.

Still the document itself, although alluded to daily in government attacks, has never been printed here.

Thus, as one critizen points out, "we are being asked to denounce en masse something that we have never read."