El Salvador's guerrilla forces hope to present Ronald Reagan with an "irreversible military situation" on Jan. 20, one of their principal leaders said today.

In a lengthy interview here, Ferman Cienfuegos, on of five commanders of the Farabundo Marti Liberation Front, said that the Salvadoran situation must be "red hot" when the new administration takes over.

Cienfuegos said the guerrillas expect to have overthown the present military-civilian coalition government by then, although the Salvadoran Army and security forces will take longer to fall.

El Salvador's four-member junta, lead by Christian Democratic President Jose Napoleon Duarte, is a relatively new and unstable form of a government that in just over a year of power has been torn repeatedly by internal dissent and rivalry.

The Army has held power in one form or another for the last 50 years and has several efficient fighting units. Nevertheless, there are indications that the political crisis has affected its unity as well, and a new movement called the Democratic Military Youth has sided with the antigovernment forces.

[Col. Adolfo Arnoldo Majano, a liberal Army officer who went underground early this month after being ousted as the junta's fifth member, charged today that government forces participated in right-wing terrorism. News services, quoting Majano at a clandestine news conference in San Salvador and in a Mexical press interview, said he called for "the present government (to be) removed by whatever means are necessary."]

The opposition to both the junta and the military power structure has united to form a political organization known as the Democratic Revolutionary Front and a unified guerrilla command.

Cienfuegose announced that both groups have agreed on at least one member of the future revolutionary government. He is Guillermo Ungo, leader of the National Revolutionary Movement and a member of the Socialist International. Ungo was a member of the reform-minded junta formed after the overthrow of Gen. Carlos Humberto Romero on Oct. 15, 1979.

Ungo resigned from that junta a year ago, charging that it was incapable of ending repression. He has spent the last several months at the United Nations lobbying for international support of the Democratic Revolutionary Front.

Cienfuegos said a guerrilla-led government would follow a seven-point program including international non-alignment, creation of a new, "honest" Army, religious freedom and human rights.

Asked whether the Salvadoran guerrilla movement is particularly hostile to the incoming U.S. administration, Cienfuegos said any U.S. president could improve relations with the Salvadoran left.

"How can the State Department expect us to say friendly things about it when it is giving support to a regime responsible for endless genocide?" he asked. "The terms of our relationship can be changed the moment the U.S. puts an effective end to all military assistance, in whatever form, to the ruling junta. What we ask of the United States is what we ask of all governments: Hands off El Salvador."

The same policy applies to Cuba and Nicaragua, Cienfuegos said. "We know the dangers involved in third-party participation in revolutionary movement," he said. "That endangers everyone." Officials of the Carter administration and advisers to President-elect Ronald Reagan have charged that Cuba is aiding the guerrillas.

The battle plans for the guerrillas' long-anticipated final offensive fell into government hands several weeks ago, but their strategy has not changed as a result, Cienfuegos said. He gave no precise date for the start of the offensive.