El Salvador's founding guerrilla faction, the Popular Liberation Forces, said today that the guerrilla movement had suffered an "irreparable" loss with the suicide of the faction's leader, Salvador Cayetano Carpio, but called on guerrilla supporters to "redouble energies . . . and close ranks" in the wake of his death.

The suicide last week of Carpio, 63, the country's first major guerrilla leader and one of its most powerful, was confirmed today in a Liberation Forces communique. According to the statement, he killed himself April 12 in Managua, Nicaragua, after learning that the murder six days before of his second-in-command was plotted by another of his most trusted associates.

His death was announced last night in Managua, where a statement by the Nicaraguan Interior Ministry said it had withheld the information for a week at the request of Carpio's family and the Liberation Forces' remaining leadership.

Carpio's unexpected death appears to be the culmination of an odd sequence of events that a State Department spokesman today likened to "gang warfare" and that the guerrillas, without elaboration, called "diversionary maneuvers instigated by the U.S. Central Intelligence Agency."

Carpio, known by his nom de Guerre Marcial, represented the oldest and in many ways the most dogmatic of the five guerrilla factions fighting to overthrow the U.S.-backed government here. Although he is said by Salvadoran leftist sources to have become increasingly cooperative with the other groups during the past six months, Carpio and his Liberation Forces have long represented a major obstacle to the tighter unity that the other guerrilla groups have sought to attain since first pulling together into a loose umbrella organization, called the Farabundo Marti National Liberation Front in early 1980.

Carpio's death and that of his second in command, Melida Anaya Montes, who was widely viewed as a moderating influence on her chief, come as rebel sources here have been telling reporters they are increasing efforts to create a more unified structure for their overall organization. There has been some talk of designating a single commander of the war effort instead of continuing with the system that gives the five group leaders equal power in a joint command.

The guerrilla communique said that the accused "intellectual author" of the initial killing perpetrated a "traitorous and nefarious action" intended "to resolve a resentment and alleged ideological and political divergencies" with Anaya Montes.

The leader of the conspiracy was identified by the Nicaraguan Interior Ministry as Rogelio A. Bazzaglia Recinos, 28, who is now under arrest in Managua with five other members of the Popular Liberation Forces allegedly involved in the murder. The guerrilla statement said that under the pseudonym Marcelo, Bazzaglia had been a member of their central command.

Anaya Montes was killed in a home outside of Managua. The Interior Ministry reported that her throat was slashed and she had more than 80 stab wounds.

The leaders of the Popular Liberation Forces, with a hitherto largely unknown figure named Salvador Guerra signing the communique as a "substitute" for commander Carpio, called on all the members of their organization to "comply" with accords reached among the leaders of the five insurgent groups "to fortify and consolidate unity."

One of the more moderate political leaders allied with the guerrillas said on a trip outside Central America that "while there will be a little uneasiness until things settle down" in the wake of the deaths, "up to now, the reaction from the leadership has been to close ranks.

"In some ways you can say Carpio's departure from the scene would be beneficial, but in other respects it is a real loss," the politician said. "Although we can say he was a very stubborn person, that same stubbornness many times helped all of us to think about what we were going to do, pointing to essential things in the struggle."

Carpio was often called, and sometimes seemed to style himself, the "Ho Chi Minh of Central America." He was an admirer of Fidel Castro and was invited by the Cuban leader to address a party congress in 1980.

Carpio, like Nicaraguan Interior Minister Tomas Borge, was an advocate of "prolonged popular war." Carpio's approach to revolution presses for sometimes slow but always relentless progress toward the establishment of a Marxist-Leninist state and society.

In the late 1960s Carpio, who had risen to prominence as a militant union leader and eventually secretary general of the Salvadoran Communist Party, broke with his former comrades rather than follow the Moscow-dictated demands that the Communists work through peaceful means in this region and that era of detente. Since then, the development of El Salvador's guerrilla movement has been marked by bitter, sometimes deadly factionalism.

Not long after Carpio founded his group, another called People's Revolutionary Army (ERP) sprang up in the wake of a frustrating national election fraud in 1972. In 1975, some of its most prominent ERP members split with that group over questions of strategy, forming a third group, the Armed Forces of National Resistance (FARN). The angered ERP leadership then "tried," convicted and executed FARN founding member Roque Dalton, an event that still causes bad feelings among the various factions.

Another group called the Central American Revolutionary Workers' Party formed, and finally even the old Communist Party that Carpio had rejected decided to take up "armed struggle" with its own guerrilla group.

As these groups sought to take advantage of the revolutionary momentum created by the 1979 Sandinista triumph in Nicaragua, they spent almost all of 1980 seeking unity, reportedly with extensive advice from Cuba and Nicaragua but never with complete success.

Of the latest deaths, State Department spokesman Alan Romberg said in Washington, "It's pretty clear that gang warfare has broken out among the insurgents." Pressing a Reagan administration theme of coordinated action among Central American guerrillas and Nicaraguan support for the Salvadorans, he also noted that "all this seems to be going on in Nicaragua."

Only three days before his death, Carpio told the crowd at the Anaya Montes funeral that "one people's solidarity with another" is nothing of which to be ashamed. Reportedly looking tired and ill, Carpio said, "The Central American people's struggle is one single struggle."