''I am sure he wanted to help,'' Mr. Michnik said in a letter. ''But, since I refused to meet him, he cost me two weeks in solitary confinement, that being the price of spurning a conversation with the representative of the U. N. Secretary General. So now, please, let no one help me in this matter. I don't know if my health could stand it.

''Neither will I accept amnesty because I do not feel guilty of anything. I demand - and will demand - an open trial and I am strong enough to wait for acquittal until such a time when Polish justice will not be disgraced by criminals in military uniforms.''

The first letter, written two months after his arrest, assessed the Government's declaration of martial law as an act of self-preservation.

''On the night of Dec. 12, the Communist elite decided desperately to defend its position as a ruling class,'' he wrote. ''Its status as an elite had become endangered not only in Poland but in the whole Communist bloc. The December military coup was not intended to revive the idea of a Communist utopia. It was an antiworkers' counterrevolution, organized in the name of the conservative interests of the ancien regime.''

Letters Reflect Tone Of Old Intelligentsia

The assessment has the tone of the old left intelligentsia, a heritage that Mr. Michnik acquired through birth. He is the son of Osias Szechter, a prewar Polish Communist who spent time in prison. He uses the name of his mother, Helena Michnik, a historian.

Mr. Michnik, who studied history at the Universities of Warsaw and Poznan, was arrested the first time during student protests in 1968 and again in 1977 with Mr. Kuron and eight other KOR organizers who were establishing links to workers' groups.

By August 1982, Mr. Michnik and the 10 others were moved from the Bialoleka detention camp to Mokotow Prison. At Bialoleka, there were throngs of detainees in temporary quarters and a great deal of coming and going. It was not difficult to smuggle letters out.