SAN SALVADOR -- El Salvador's Marxist guerrillas, in a shift of strategy, are stepping up activities to foment an "insurrection" in the capital and prepare for a national "strategic counteroffensive" in the next two years, according to rebel documents said to have been captured by Salvadoran security forces.

The documents, according to the security forces, were captured in raids in April on "active safehouses" of the Farabundo Marti National Liberation Front (known by its Spanish initials FMLN).

The documents appear to reflect the implementation of a previously announced rebel objective of intensifying insurgent activities in the capital.

The FMLN internal documents were captured in late April in raids by security forces on rebel safehouses in Ayutuxtepeque, on the northern outskirts of San Salvador, and in Texistepeque, north of the town of Santa Ana in the eastern part of the country, according to the Salvadoran Armed Forces.

The raids in which the documents were seized were reported at the time and sketchy accounts of their contents have appeared in the local press.

Copies of some documents and transcripts of others from the two caches were shown to two Washington Post reporters on separate occasions last month and earlier this month. They were allowed to review them and take notes on their contents, but no photocopies were permitted.

Among the more recent documents were handwritten letters that employed code words and numbers when referring to specific organizations, persons and places. Included in the caches were keys to be used by the rebels to code and decode the messages.

The tracts call for a series of increasingly aggressive activities in conjunction with certain labor, humanitarian and refugee organizations that the government of President Jose Napoleon Duarte has denounced as rebel "fronts."

"The central idea is that we not remain in the mere act of mobilization . . . but pass to more aggressive actions that prepare the ground for violence of the masses," one of the documents says.

Another, entitled "The Strategic Offensive or Counteroffensive," describes efforts to reach a point at which the rebels' "political and military forces" converge to "snatch power away from the enemy."

It talks of combining military actions by FMLN guerrillas with "insurrectional" activities by civilian organizations and outlines plans for intensified labor strikes, sit-ins, marches, land takeovers, infiltration of the armed forces, protests in front of the U.S. Embassy and the presentation of specific demands to the government and public.

The documents outline plans involving the participation of several groups that are not mentioned by name.

Among the groups described by the government as rebel front organizations are the National Union of Salvadoran Workers; the Committee of Mothers of Political Prisoners, the Disappeared and the Assassinated; the Committee of Families; the Committee of Political Prisoners of El Salvador; the Christian Committee of the Displaced of El Salvador and the Committee of the Dismissed and Unemployed of El Salvador.

Rebels and their supporters in the past have denounced as fakes documents purported to have been captured by the military.

Especially controversial was a 1981 State Department white paper that cited purportedly captured rebel documents in charging that human rights groups had been infiltrated by the rebels and that the guerrillas were receiving arms from Nicaragua's Sandinista government.

Nicaraguan President Daniel Ortega, in an interview with the Mexican newspaper Excelsior, recently acknowledged that weapons had been sent to the Salvadoran rebels for a time shortly after the Sandinistas took power and that some supplies for the FMLN may still be passing through Nicaraguan territory without the government's knowledge. But he denied that Nicaragua currently had any policy of supplying the Salvadoran rebels.

None of the recently captured documents that were seen by The Washington Post make any reference to foreign arms supplies.

Supporting assertions that the documents are genuine is the fact that they outline a program of actions that indeed took place after the caches were seized.

A member of the Democratic Revolutionary Front (FDR by its Spanish initials), which is allied with the FMLN, did not dispute the papers' authenticity this month when shown a transcript of one key document from the FMLN's general command. But the member expressed concern that the guerrillas were "overestimating" their capabilities and closing off opportunities for a political solution by developing plans for an "insurrection."

Two organizations mentioned prominently in the documents, the National Union of Salvadoran Workers and the Committee of Mothers, emphatically denied that they have any links with the FMLN.

The workers' union, a leftist labor federation that includes student, peasant and refugee groups, issued a statement June 2 denouncing a "dirty and criminal action of the government" to portray "our organizations as fronts of the FMLN-FDR." The federation said it "rejects this accusation as a worn-out argument of the government."

Marlene Perez, a spokeswoman for the Mothers Committee, which is known as Comadres, told Washington Post correspondent Julia Preston, "We don't have any relationship with the FMLN. Our work is neutral."

Perez charged that information linking organizations such as Comadres to the FMLN came from false confessions extracted from political prisoners under torture. She said she knew nothing of the captured documents, which she described as "Armed Forces calumnies."

Comadres, which was formed in the 1970s, has won international attention -- including the Robert F. Kennedy Human Rights Award -- for its work in denouncing right-wing death squads.

Of the more than 60,000 Salvadorans said to have been killed since the civil war began in 1979, an estimated 40,000 are believed to have been murdered by death squads.

The committee's headquarters has been the target of several attacks, the latest of which -- a bombing by unknown perpetrators on May 28 -- wounded two women members and damaged the committee's offices here.

One seven-page section of the documents on "the activities of Lil," a code word identified in a key as referring to Comadres, states: "The Lil have imbued their latest activities with a great aggressiveness and violence . . . but we must orient them so their steps will be greater and practiced." It adds, "We are launching the masses to more violent activity. We cannot be irresponsible. We must endow them with instruments." There is no explanation of what is meant by this term. This document does not carry a date or title but includes plans for the first quarter of this year.

In outlining orders from "ZZ," which is identified in a code key as the FMLN "general command," this document states, "We must proceed in testing or creating situations that are at first preinsurrectional. ZZ has determined that the next two years are to prepare the strategic counteroffensive . . . ."

It adds, "ZZ has approved a plan to generate an opportunity and realize a test of the strategic offensive." The objective, it says, is to "promote simultaneous activities of the masses and military pressure."

The document then describes plans for the "hotel sector," identified in a key as the "metropolitan zone."

The plans call for the occupation by Comadres and an organization of war refugees -- the Christian Committee of the Displaced of El Salvador -- of churches, cathedrals and an unnamed embassy, the seizure by protesters of a street in front of the National Assembly for two hours, marches in which slogans are painted on streets, buildings and vehicles and a prison "riot" by members of the Committee of Political Prisoners of El Salvador in which hostages are taken.

The document goes on, however, to register the author's objection to a prison riot with hostages because "we don't have a cadre inside capable of conducting this situation."

Instead, the author calls for a demonstration at the gates of the Mariona Prison in support of political prisoners held there. Another document, a handwritten note dated April 22 ordering a "Comrade Carlos" to pass on instructions from "superior organs," mentions plans for actions at the Mariona Prison and a women's prison.

In response, Preston reported, a 12-page Salvadoran Treasury Police analysis, dated April 29 and marked "confidential," urged immediate transmission of the documents to the military high command and recommended intensified security at embassies and the National Assembly.

It also called for increased discipline to "prevent the armed forces from falling into the trap of causing deaths in the activities these organizations hope to carry out, since their goal is to denigrate" the armed forces.

On May 31, the National Union of Salvadoran Workers and affiliated organizations staged a demonstration outside Mariona.

The protest, to demand a general amnesty for political prisoners, resulted in a controversial shooting incident in which a member of the union's executive committee, Julio Cesar Portillo, was wounded in the back by a .22 caliber bullet, the source of which has yet to be determined.

Leaders of the demonstration charged that the shot was fired by prison guards, but the government argued that security forces do not use such small-caliber guns and that the shot came from within the crowd.

In June, groups of war refugees and members of Comadres began a series of takeovers of churches and the San Salvador Cathedral that appeared to coincide with the FMLN plans.

The biggest sit-in, by about 35 members of Comadres, began June 6 in the cathedral in support of three women protesting the May 1 disappearance of their sons, who were leaders of a leftist coffee-pickers union in Santa Ana Province.

The Roman Catholic archbishop of San Salvador, Arturo Rivera y Damas, denounced the action as "unnecessary and intolerable," said it was linked to protests by groups sympathetic to the FMLN and threatened to excommunicate the women, who later called off the sit-in.

Violence broke out between security forces and protesters in San Salvador Wednesday when about 100 members and supporters of a Social Security workers union, some of them armed with clubs, tried to occupy the Institute of Social Security building and clashed with police guarding the site, news agencies reported.

When police outside the building fired in the air to disperse the crowd, police inside apparently thought they were under fire and began shooting wildly, the news agencies said.

About 20 persons were reported wounded, including protesters, several policemen and two Salvadoran journalists. The Social Security workers union is a member of the National Union of Salvadoran Workers.