A young Latvian Baptist being held in a Soviet prison camp has addressed an appeal for help to the international disarmament conference here. In the July-dated letter, Janis Rozkalns, who was sentenced in November 1983 to five years imprisonment and three years exile for religious and political activities, charges Soviet prison authorities with deliberately starving and freezing prisoners in transit to labor camps and with denying them medical help.
He describes a stopover of his prison transport at a jail in Pskov, a town about halfway between Riga and Leningrad: ''The barred windows of the cell were broken, and it was dirty. In the middle of the cell - an open toilet. There was a draft and water dripping.
''When we woke at night, suffering from insect bites, our boots were afloat in water. Leeches crawled on the floor, there were bed bugs and centipedes on the walls. It was freezing outside, but we slept without blankets.''
A few days later, Mr. Rozkalns relates, he and the other prisoners were moved to a cell in which there was leaking hot water. After some 10 days in the steaming cell, the prisoner transport continued. Rozkalns was put in a drafty car and fed only bread and water.
By the time he reached a prison in Perm, Rozkalns says, he was seriously ill.
Rozkalns writes that on March 10 a fellow prisoner named Oleg Shevchenko suffered an apparent heart attack. The officer in charge of the guard refused to summon medical help.
He says that when Shevchenko was unable to rise from his bed the following day, prison authorities sentenced him to 15 days in a cold cell, with meals only every other day.
Rozkalns is the 35-year-old father of twins, who were born three months before his arrest in April 1983.
His appeal, smuggled to Latvian emigres in Europe, was translated by the World Federation of Free Latvians for the fourth session of the Stockholm Conference on Security-building Measures and Disarmament, which opened Nov. 6.