At Mokotow, the situation was more difficult. The political prisoners were kept apart, each one sharing a cell with inmates convicted of common crimes. Visitors were closely searched.

But at least some of Mr. Michnik's letters got through. In May 1982 he wrote:

''I am one of those who for the last 10 years criticized the concept of conspiratorial activity. Today, I am for forming the underground. We have no choice. The choice was made for us by Jaruzelski. This is what Polish honor and the Polish mind command.

''But let me say at the beginning what the underground should not be. It should not be an underground state, with its national government, parliament and military power. It cannot be an underground state for it does not have the national mandate for it.''

This reference to a secret state is immediately understood by Poles as an allusion to World War II when an underground administration functioned under the Nazi German occupation.

Michnik Tried to Avoid A Direct Confrontation

In his longest letter - more than 4,000 words - Mr. Michnik developed his ideas of building social institutions outside official structures while avoiding direct confrontation.

''Jaruzelski and his generals will pay any price to keep power,'' he said. ''That is why, in my opinion, it is the order of the day to abandon thinking of a radical strike at the center of power. It does not offer any chance of success whatsoever and makes a massacre very probable. It is our duty to avoid Soviet intervention at all cost. The patriotic slogan of the moment is to live for Poland, not die for it.''