An open letter signed by 15 political prisoners has triggered a wave of public protest over possible violations of human rights in Brazil.

In a document published in the nation's two leading newspapers late last month, the 11 men and four women - all recently arrested on charges of violating Brazil's detailed national security laws - charged that they were systematically tortured by army police in Rio and Sao Paulo. They have since repeated their charges in several widely publicized court hearings.

Three of the women are also among the organizers of a prison hunger strike that is now more than two weeks old. The hunger campaign, which has been joined by 33 other political prisoners in Rio and an estimated 30 in Sao Paulo, is in protest against conditions for female political prisoners and death threats the women say they have received.

What's more, the heating up of the political prisoner issue here coincides with the visit of U.S. Export-Import Bank President John Moore, whose agency recently announced that respect for human rights will be one of the criteria considered in the evaluation of future Ex-Im-Bank loans.

Moore, who arrived last week to meet with businessmen and government officials, has so far refrained from specifically criticizing the human rights situation in Brazil, which has Ex-Im-Bank loans currently total $1.7 billion.

"I can say something only after I've received official State Department reports on the subject," Moore said early this week.

Human rights have been a subject of disagreement between the United States and Brazil since the Carter administrator took office. Larlier this year, the Brazilian government cancelled a military agreement with the United States after Washington released a report critical of the human rights situation here.

The latest denunciations of alleged human rights violations have been seconded by various religious groups and by the Brazilian National Lawyers Association, which has also renewed its call for the restoration of nabcascurpus.

THe prisoners' letter has also sparked protests from the Democratic movement. Brazil's only legal opposition party. Movement leaders called the situation described in the letter "a festival of barbarism and sadism, an authentic spectacle of bestiality."

The letter has been described by the nation's leading newsmagazine as "possibly the most important document ever published in Brazil with denunciations of cruelties suffered by political prisoners "and public reaction to it has been especially strong because the document appears to disclose the existence of several previously unheard-of torture devices.

Chief among these is "the icebox," described as "a refrigerated cement both, soundproofed and placed inside a larger, hermetically sealed compartment of concrete." Prisoners are said to be exposed to freezing temperatures over long periods, doused with cold water and subjected to "Strident sounds of a high frequency" which induce "nausea, vomiting and even madness."

The prisoners also claim that police make frequent use of "the dragon." This is described as a "barber's chair" in which prisoners are restrained with "leather straps that are padded so as not to leave marks" and then subjected to electric shocks with wires connected to "a small instrument with a handcrank that increases the intensity of the current."

One male prisoner also said that he was forced to undergo a torture known as "the cross." He says that while he was pinned against a wall, with his arms forced apart and his legs held together, policemen kneed him in the genitals, plucked out his public hairs, and threatened that he would die "just like Jesus."

In addition, both male and female prisoners charge that they were sexually assaulted by officers in the Rio headquarters of the army's Department of Internal order (DOI). They, described unit's offices as "veritable scientific laboratories of torture," and suggest the existence of an extensive "torture infrastructure."

According to the prisoners, officials in Rio claimed to be "exporting tecnology" and boasted of the efficiency of their torture devices. "Here there are no deaths as in Sao Paulo," one of the policemen is alleged to have said, in a reference to the death two years ago of journalist Vladimir Herzog in the army unit's headquarters in Sao Paulo.

Presidential press secretary, Jose Maria de Toledo Canargo, who recently said "there are no political prisoners in Brazil," described the prisoners' charges as "serious and impressive." He said that the government of Gen. Ernesto Geisel, which has been trying to acquire a more moderate image abroad and at home, would make a thorough investigation. Army officials in Rio and Sao Paulo, however, immediately denied that the prisoners had received any "inhumane treatment."

Since the beginning of the hunger strike, some of the prisoners have been transfered to hospitals or release from preventive detention, an action they say was taken merely to "deflate" their protests. The hunger strike continues, however, under the leadership of Rosalice Migaldi Fernandes pereira, an opposition party representative in the Rio de Janciro state assembly who is currently serving an 18-month term for having distributed "trotskyite" pamphlets to workers.