More than 2,000 Guatemalans, most of them opponents of the rightist military government, have been murdered in continuing political violence over the past 18 months, according to an Amnesty International report released yesterday.

The report by the prestigious London-based human rights group does not directly accuse the government of Gen. Romeo Lucas Garcia of responsibility for widespread and increasing kidnappings, disappearances, tortue and assassinations. But it notes that most of the victims have been "trade unionists, peasants, students and academic leaders, church and democratic political" activist who have pressed the government for reforms.

"In no case of which Amnesty International aware," the report notes, "has a killing been fully investigated and its perpetrators brought to justice."

The Amnesty report, released in the form of a memorandum presented to the government of the Central American country following a five-day investigation there in August, comes at a time when some State Department officials have sought resumption of U.S. military aid to Guatemala.

The military aid programs stopped in early 1977, when Guatemala rejected them to protest that Carter administration's cut-off of development aid to other Latin American countries on human rights grounds.

An administration proposal last spring for a new $250,000 military training program for Guatamala was killed in Congress.

In a visit to Washington last month, U.S. Ambassador to Guatemala Frank Ortiz maintained that the human rights sdituation there had improved, according to official sources. Ortiz said that the Lucas Garcia government needed increased armament and U.S. support to avoid a Nicaraguan-styl revolution.

The State Department human rights office has maintained that there is no improvement in the Guatemalan situation.

The most powerful member of a group of rightest military government that historically have dominated Central America, Guatemala has become increasingly isolated in the region following the overthrow of governments in Nicargua last July, and El Salvador last October, and their replacement with leftist and left-center groups.

In previous reports, Amnesty International has charged that more than 20,000 Guatemalans have been killed since 1966 by right-wing paramilitary death squads who operate "with complete impunity."

The report released yesterday cited particular repression against trade union leaders who the government and the economic elite that supports it feel will upset the existing economic system.

"To be a union leader in Guatemala today means risking one's life," the report said. Since July 1978 under the Lucas Garcia government, it said, 12 union leaders were assassinated and nine kidnaped.

The report notes "persistent allegations" of electoral fraud and "obstacles" placed before opposition political organizations, "ranging from long delays in recognition of parties to political murder." It cites the Jan. 25 murder of social democratic leader Alberto Fuentes Mohr and the March 22 killing of populist leader Manuel Colom Argueta, both "a few hours before" their parties were to present requests for official registration and permission to participate in elections.

Amnesty recommends, "as a matter of urgency," that the government respect and enforce constitutional guarantees of free political and labor organization, and that it "fully and fairly" investigate charges of human rights abuses if it "hopes to restore the faith of its own citizens and that of international public opinion in its ability. . . to enforce the law."