A key civilian in El Salvador's fragile coalition government resigned last night, raising speculation that the second U.S.-backed junta within six months is on the brink of collapse.

The departure of Christian Democrat Hector Dada, one of three civilians who formed the junta with two Army colonels, comes as violence by extremists of the left and right is approaching the proportions of full-scale civil war.

Meanwhile, violence in El Salvador increases daily.

According to church officials there, more than 600 people have been killed since the beginning of the year. In recent days, dozens of mutilated and machine-gunned victims of right-wing terror have been found in San Salvador and in the countryside.

At least three major clashes between leftist guerrillas and the armed forces were reported Monday night, including an attack by the People's Revolutionary Army on the main garrison of the National Guard in San Salvador. The guerrillas used cannon and bazookas in the assault before finally being repelled. Four guerrillas and one guardsman reportedly were killed in that action.

The Christian Democrats have scheduled a Sunday meeting to select a new junta member. But sources contacted by telephone in San Salvador, the capital, expect an open split between those who believe the party should continue its participation in the civilian-military junta, and those who want to quit.

Civilians in the first junta resigned in January, accusing the military of failing to permit promised economic, social and political reforms. The U.S. government has described the Christian Democrats' participation in the junta as El Salvador's "last hope" for averting civil war. The party formed a new junta in January on the basis of a reworked reform "pact" with the military but Dada reportedly charged that the pact had been broken.

In an unpublished letter of resignation to the junta and party, sources said, Dada accused the military of failing to allow promised land and economic reforms, and with increasing repression against the left and popular organizations.

Observers said other cabinet resignations may follow Dada's. Speculation also increased over the possibility of a right-wing coup, with rightist military factions backed by El Salvador's conservative economic elite attempting to take over before the junta can be reconstituted again.

A rightist coup was narrowly averted less than two weeks ago, when the United States heavily pressured the right by threatening withdrawal of military, economic and diplomatic support.

A debate over what to do about the Salvadoran situation is under way in the State Department. One faction contends that the official security forces must be propped up to secure public order before needed reforms can be implemented. Others say that the left has gained substantial popular support by advocating reforms and blaming the junta and military for lack of action -- and that the left itself must be brought into the government. l

The most likely Christian Democratic candidate to replace Dada in the junta is Jose Napoleon Duarte, who won the presidency of the Central American country in 1972 only to see the election results voided and a general put in office.

Duarte, who was once considered the most popular politician in the country, has been unable to muster widespread support since he returned from seven years of exile at the end of 1979.

Some young Christian Democrats are saying that if the party continues to support the current junta by appointing Duarte, they may break from it altogether.

The United States has been encouraging the remaining liberal factions of the junta to start the reforms that have been promised since October.

Several sources in El Salvador report that the redistribution of land may start this week, although continued violent opposition is expected from the wealthy elite.

The advocates of reform, including Christian Democrats within the government, increasingly have become the object of attack.

Less than two weeks ago, Mario Zamora Rivas, a Christian Democratic Cabinet member from a politically influential family was machine-gunned to death in his home. Party leader Duarte cited that killing as a key factor in Dada's resignation. Zamora had been denounced by extreme rightists before his death.

Yesterday, the Christian Democratic leader in El Salvador's Morazon district, Daniel Escobar, was also assassinated.

An air of desperation is apparent among the reformists in the current regime. One of them said of Dada's resignation, "Maybe he just got tired of talking and haggling. I think he just lost heart.

"The killing hasn't stopped. Everybody is on the blacklist and now it's a matter of pushing as fast as we can for the reforms. But we're losing people day by day through threats, direct or indirect, or direct elimination. I don't blame Dada at all."