SAN SALVADOR, NOV. 5 -- President Jose Napoleon Duarte declared a unilateral cease-fire today in the government's eight-year-old war with Marxist-led guerrillas and called on Soviet Bloc and other countries to stop aiding the rebels in accord with a Central American peace plan taking effect today.

"The armed forces will suspend offensive operations starting Nov. 5, 1987, in the entire national territory," Duarte said in a speech on his government's compliance with the regional peace plan.

The plan, signed Aug. 7 in Guatemala by the presidents of Nicaragua, El Salvador, Honduras, Guatemala and Costa Rica, calls for simultaneous cease-fires, amnesties, democratization measures, halts to foreign aid for insurgencies and prohibitions against rebel bases in neighboring countries.

In addition to the cease-fire, Duarte announced measures to encourage rebels to accept amnesty, such as medical assistance and humanitarian aid. The government plans to start releasing at least 424 accused rebels from prison Friday under a new amnesty law, and government officials estimated that as many as 750 eventually may go free.

Duarte said he had ordered the unilateral cease-fire because the Farabundo Marti National Liberation Front (FMLN), a coalition of five guerrilla groups, and the Democratic Revolutionary Front (FDR), a political opposition group allied with them, had broken off talks on a negotiated truce.

Government and rebel representatives held peace talks here Oct. 4-5 and met again in Venezuela to try to negotiate a cease-fire, but the rebels refused to attend a subsequent meeting scheduled for last week in Mexico to protest the Oct. 26 murder of a leftist human rights commission leader.

"Once again we encounter the lack of will and political sincerity of the FMLN-FDR, who clearly show with this attitude their desire for violence and war," Duarte said.

The Christian Democratic leader said that as part of the cease-fire, the armed forces would suspend the use of artillery and attack aircraft unless specifically authorized for defensive actions. Duarte said the military would carry out "actions of protection and security" in conflict zones with units permanently based there, but he did not elaborate.

Duarte called on the FMLN to stop all military actions, economic sabotage, terrorism, assassinations, extortion and kidnapings of military and civilian officials.

"This is a message to all those in arms to come down from the mountains, find the places to turn yourselves in, deliver your arms and take up civilian life," Duarte said.

There was no immediate response from the rebels, but they continued to enforce an apparently effective transportation strike for the third day today to protest the killing last week of Herbert Anaya, the head of the leftist Human Rights Commission of Salvador.

Guillermo Ungo, the head of the FDR, said from exile in Panama that Duarte's speech was a propaganda ploy. He added, "This is a cease-fire for show. No one will be fooled by this trick. It cannot work."

Citing the peace plan's prohibitions against foreign aid to insurgencies and the use of one state's territory to "destabilize" another, Duarte called on Nicaragua, Cuba, Libya, East Germany, Vietnam and the Soviet Union to "respect" the accord and "stop aiding the FMLN-FDR." The government has long accused Cuba and Nicaragua of providing military and logistical aid to the rebels, and U.S. officials say that some captured rebel weapons have been traced to U.S. stocks left behind in Vietnam, but the rebels deny receiving outside military support.

Duarte also called on countries with "permissive" policies on the presence of foreign dissident groups -- he cited Mexico and Panama -- to "prevent the activities of the FMLN-FDR against the stability of our government and invite them to leave your territory."

He said his government would continue to observe a policy of "nonintervention in the affairs of other states," making no mention of disclosures of U.S.-run operations from El Salvador to support Nicaraguan rebels, notably the use of Ilopango air base for supply flights.