Czechoslovak authorities are tightening their grip on dissident intellectuals by moving against what has been one of Eastern Europe's most active underground literary movements.

In what is described as the first arrest for literary rather than political activity of a writer, the government last week jailed Jiri Grusa, 40, on grounds that his latest novel is "inciting anti-socialist" feelings.

The arrest was revealed in an open letter made available here yesterday in which a group of 20 prominent dissident writers accused the government of breaching an unwritten understanding which has allowed the nonconformist underground literary movement to flourish in Czecholovakia.

Grusa is one of more than 100 Czechoslovak writers whose works have been blacklisted by the authorities since suppression of the 1968 liberal movement known as "Prague Spring."

Instead, they have circulated their works through a remarkable organization popularly known as Padlock Press, an underground publishing system which has issued nearly 150 new titles during the past four years, some of them in hard bindings.

According to the letter, Grusa and another Padlock member arrested for having distributed the typewritten manuscripts, face criminal charges that carry a penalty from six months to three years in prison.

The letter was signed by Czechoslovakia's best-known literary figures including Vaclav Havel, Pavel Kohut, Ludvik Vaculik and Ivan Klima.

Until last week, Padlock Press was quietly tolerated by the czechoslovak authorities - even though its activities were viewed with considerable dismay. Now, claim the signatories, the government "has overstepped the thus-far quietly respected dividing line between political activities which a number of writers have been involved and literary activities which people like Grusa are engaged in."

The letter described Grusa's pending trial as "the first trial (in Czechoslovakia) of an artist literary work, a rare spectacle even in the illiterate countries of the world."

The signatories are concerned that Grusa's arrest could be an attempt to intimidate other writers associated with Padlock Press. They argue that the plot of his novel, which is based on the practice of asking job applicants to fill out lengthy personal questionnaires is not primarily political.

"Perhaps Grusa was chosen as a suitable target because he is thus far unknown in the world and therefore on one will stand up for him. But that is a mistake because soon he will be very well known.His book "Questionnaire" is being prepared for publication by the Reich Publishing Company in Lucerne, Switzerland, and by the Gallimard Publishing Company in France," the letter continues.

The moves against Padlock Press coincided with a round up of over 50 supporters of the human rights move-week of Soviet President Leourd ent Charter 77 during the visit last Brezhnev. But most of those detained have now been released.

Some Czechoslovak human rights activists interpret the latest police action as a sign of nervousness prior to the 10th anniversary of the Soviet invasion on Aug. 21, 1968. That quashed the "prague spring." They link it to a Voice of America broadcast to Czechoslovakia last month which reported that Padlock Press intended to mark the anniversary by publishing a collection of articles by leading Czechoslovak writers.