The Inter-American Human Rights Commission has declared the government of Guatemala responsible for a "great majority" of the "thousands of illegal executions" that have occurred in that Central American nation in recent years.

In a 134-page report distributed yesterday, the seven-member commission concluded that the military government -- which the Reagan administration has vowed to support through resumption of military aid -- has "instigated or tolerated" an "alarming" level of violence generally.

The commission is an autonomous agency of the Organization of American States. Its reports on rights violations have particular impact in the Western Hemisphere as a judgment by peers. This impact is enhanced because of the standard practice of offering the accused government exhaustive opportunity for rebuttal.

Guatemala, despite occasional declarations that it had nothing to hide, refused repeated efforts by the commission to make an on-scene investigation. As detailed in the report, the commission also received scant cooperation through official exchanges that resulted from denunciations filed by the families of victims.

As a result of its compilations, however, the commission found that those assassinated in Guatemala "are for the most part leaders of opposition political parties, union members, priests, lawyers, journalists, teachers, and thousands of peasants and Indians" killed by "security forces or groups of paramilitary civilians acting with the close collaboration of government authorities."

Noting the absence of any prosecution of rights violators, the commission recommended "total reorganization of the judicial system," giving it the power and resources to prosecute "without fear of reprisals" those responsible.

A spokesman of the Reagan administration, retired Gen. Vernon Walters, discounted rights violations in Guatemala when he made a visit there in May. The U.S. government sought congressional repeal of strictures against military aid but encountered resistance because of Guatemala's record of rights violations. Nevertheless, a sale of military trucks was made in June.

A State Department official said yesterday that the administration agreed not to attempt further military aid without consulting Congress and "no formal consultations" are now under way.

The OAS rights commission also issued a detailed report yesterday on massive, continuing violations in Bolivia. The United States has refused to deal with that military government mainly because of its alleged ties to the drug trade as well as its human rights performance.