Deep fissures appear to have developed between El Salvador's Marxist guerrilla leaders and the more moderate politicians allied with them, as the politicians have begun to criticize publicly guerrilla attacks on civilian targets.

The disputes reinforce the perception that the guerrillas fighting in the field often operate independently of their civilian allies, such as Guillermo Ungo and Ruben Zamora, who live in exile.

Current disagreements appeared likely to fuel longstanding charges by critics of the Salvadoran left that the politicians lack significant influence over the guerrillas, Salvadoran and U.S. political observers said.

The politicians travel widely as spokesmen for the Salvadoran rebel movement, and they are better known abroad than the reclusive guerrilla commanders who direct the fighting from remote strongholds in El Salvador's northern mountains.

Critics of the left, including the Reagan administration, say the politicians are not legitimate representatives of the rebel movement because the military leaders are the ones who wield real power. In this view, the politicians help give the left a more moderate image than it deserves.

This view is similar to the administration's criticism of Nicaragua's Sandinistas, who were the military commanders in the insurrection that toppled the Somoza family dictatorship in 1979.

The Sandinistas gained international support for their struggle in part because they were allied with moderate businessmen and politicians. Since 1979, however, many of these former allies have become disillusioned because of their lack of power in the government and have turned against the Sandinistas. Many have joined the U.S.-backed rebel movement fighting to overthrow the Sandinistas.

The politicians in El Salvador's rebel movement have had differences with the guerrillas in the past but generally have kept them private. Since the spring, however, leftist civilian leaders have criticized the guerrillas publicly for burning town halls and, most dramatically, for attacking a row of sidewalk restaurants in San Salvador June 19. Four U.S. marines and nine civilians, including two Americans, were killed in that attack.

Ungo and Zamora issued formal statements objecting to the restaurant attack. But top-ranking guerrilla leaders, at a rare meeting with U.S. correspondents in rebel-dominated territory last month, brushed aside these objections.

"It is completely normal that in some situations there can be different opinions," said guerrilla commander Jorge Shafik Handal.

U.S. military officials and diplomats who follow the situation in El Salvador described the disputes as an opportunity to drive a wedge between the guerrillas and their political allies. The U.S. government recently has "pointed out" to several West European and Latin American governments that Ungo and Zamora disagreed with the guerrillas on some important issues, U.S. officials said.

A full rupture seems unlikely in the near future, as left-wing political leaders said they planned to stick by the guerrillas and even suggested that they felt more comfortable voicing some criticism rather than keeping quiet. But the disputes appeared likely to diminish the credibility of Ungo and Zamora in their role as the best-known spokesmen for the Salvadoran left, said Salvadoran and U.S. political observers.

Ungo played down the differences in an interview, saying they were "similar to those in any marriage."

Ungo said that some disagreements had become open this year because the guerrillas and politicians were consulting more closely on a greater number of issues than in the past, so there were more issues to dispute. But a well-placed Salvadoran left-wing source said privately that the disagreements had weakened the left: "This is not the historic moment to be emphasizing the differences, to be making them public."

Ungo is president of the Democratic Revolutionary Front, known by its Spanish initials FDR, which includes several political parties and professional organizations that share an ideology close to that of West European social democratic parties. The FDR is allied with the Farabundo Marti National Liberation Front, or FMLN, which consists of five guerrilla forces.

The FMLN declines to call itself Marxist, but its political statements and practice of guerrilla war are firmly within the Marxist revolutionary tradition. The FMLN announced Wednesday that it intended to merge its five forces into a single guerrilla army at some point in the future, but such a union would not directly affect its relations with the FDR.

Differences have arisen in the past between the guerrillas and their political allies. Last year, for instance, senior political leaders criticized the guerrillas -- not for attribution -- for confiscating Salvadoran citizens' identity cards to prevent them from voting in the March presidential elections. The FDR also quietly urged the guerrillas to halt forced recruitment of youths, a practice that the guerrilla leadership now says was an error.

In both of those cases, the political leaders avoided acknowledging in public their criticisms. Under persistent questioning at a news conference in Mexico City a year ago, Zamora and other FDR leaders declined to condemn forced recruitment, although they were letting it be known that they were doing so in private conversations with guerrilla leaders.

Last March, however, Ungo publicly criticized the guerrillas for attacking and burning several town halls before the March 31 legislative elections. He said the attacks were having a negative political impact.

A much more important dispute came to light following the killings of the four unarmed marines, who worked as embassy guards, and other diners at a row of sidewalk restaurants. The FMLN sought to blame some of the civilian deaths on unidentified persons who supposedly fired back at the attackers, but investigators and most witnesses agreed that the guerrillas were responsible for most and probably all of the casualties.

Zamora's party, the Popular Social Christian Movement, issued a communique five days after the attack saying that it "condemns the terrorist actions" at the restaurants because the attack violated the Geneva Conventions.

In interviews, Zamora later backed off somewhat from the communique and said the marines were a legitimate military target. But his party remained on record as condemning the assault, and Salvadoran left-wing sources said the party had issued the communique in response to the civilian casualties.

Ungo waited nearly two weeks to comment, but his National Revolutionary Movement finally issued a five-point statement saying that it "lamented" the attack. The party said that it "does not share in, or approve, all of the actions that some of our allies carry out, such as the lamentable events" of the restaurant assault. But the party also made a point of reaffirming "its determination to stay in and strengthen the FDR-FMLN alliance, preserving its [the party's] autonomy and own identity."

There were several indications that even some elements in the guerrilla leadership were taken aback by the brutality of the restaurant attack. The FMLN's general command waited five days before it issued a formal communique claiming responsibility, and all of its statements focused on the killings of the marines and steered away from discussing the civilian deaths.

It remained unclear, however, whether the politicians' criticism would help deter the guerrillas from targeting civilians in the future. Ungo said that the FDR had reached "some agreements" with the FMLN on protecting the civilian population, although he did not spell them out.