El Salvador's main left-wing guerrilla organization, the Farabundo Marti National Liberation Front, today claimed responsibility for Wednesday's restaurant attack that left four U.S. marine guards and nine civilians dead.

The clandestine Radio Venceremos of the five-member rebel coalition, known by its Spanish initials FMLN, broadcast the communique that surfaced yesterday claiming the assault in the name of an urban command.

"Did Ronald Reagan think that he would come to make war in El Salvador and that his soldiers were not going to die?" the radio, based in a major guerrilla stronghold in mountainous northeastern Morazan province, said. "Urban guerrillas of the FMLN, we salute you from this antiimperialist trench."

The commando unit that staged the attack, named after dead guerrilla leader Mardoqueo Cruz, was formed last year and has about 75 members who specialize in combat and sabotage, according to a confidential Salvadoran armed forces document. About 10 guerrillas showing crisp military discipline killed the off-duty, unarmed marine embassy guards, and apparently most or all of the civilians as well, according to witnesses, and Salvadoran and U.S. officials.

The civilian dead, all men, included two visiting U.S. computer company employes; their three business colleagues of Salvadoran, Chilean and Guatemalan nationalities; a Salvadoran law school administrator; a Salvadoran student, and two Salvadorans of undisclosed occupations. "Mardoqueo Cruz Urban Guerrilla Commandos of the FMLN," whose title appeared at the top of the communique that was released yesterday, had staged four earlier attacks in the San Salvador metropolitan area, according to the armed forces document.

It is the urban front of the Central American Revolutionary Workers' Party. That group, with about 500 members, is one of the smallest in the overall guerrilla coalition whose membership is estimated at between 6,000 and 7,000.

The Mardoqueo Cruz unit ambushed a Salvadoran Army Signal Corps patrol on Oct. 23, 1984, and attacked National Police vehicles on Feb. 20 and April 16 of this year with automatic rifles, hand grenades and rocket-propelled grenades, the document said.

The document did not give the number of victims in these attacks, but Salvadoran military officials said the unit previously had not staged any attack as costly or dramatic as the one on Wednesday.

Questions remained about whether anyone, and if so who, fired back at the leftist commandos. The rebels' communique said unidentified armed persons in civilian clothes had opened fire from the periphery of the row of trendy restaurants, and it sought to blame these persons for the civilians' deaths.

Witnesses' accounts were contradictory. Some said they did not think anybody shot back, while others said that one or two persons in civilian clothes had pulled out pistols and done so. Several witnesses reported hearing individual pistol shots, which contrasted with the bursts from the attackers' Israeli-made Uzi submachine guns and U.S.-made M16 automatic rifles. Witnesses uniformly reported that firing by the attackers appeared to have been indiscriminate at times and that several civilians were deliberately shot in the legs while lying on the floor.

A U.S. Embassy official said today that he understood that fire had been returned by one or more Salvadoran National Policemen assigned to guard the Brazilian ambassador's residence across the street from the restaurants.

This was disputed, however, by the National Policeman guarding the residence this afternoon. He said he had spoken with the guard stationed at the residence Wednesday evening, who said that he did not shoot because his orders were not to attract fire toward the compound where the ambassador lives.