SANTIAGO, Chile — A judge convicted six men on Wednesday in the 1982 murder of former President Eduardo Frei Montalva of Chile, then the leader of the moderate opposition against the dictator Augusto Pinochet.

In an 811-page ruling, Judge Alejandro Madrid found that the men — a former security agent, four doctors and Mr. Frei’s driver — conspired to slowly poison Mr. Frei after he had surgery in a private clinic in Santiago, the capital, and then worked to conceal the autopsy report.

Mr. Frei, of the centrist Christian Democratic Party, served as president of Chile from 1964 to 1970. His government began a land reform program and took majority control of the copper industry, then in the hands of foreign corporations.

After initially supporting the 1973 military coup against his successor, Salvador Allende, Mr. Frei and his party soon became vocal opponents of the military junta because of widespread human rights violations. At the time of his death on Jan. 22, 1982, Mr. Frei was leading efforts to unite the moderate political opposition to oust General Pinochet.