Leftist guerrillas declared a unilateral 24-hour truce today to mark the anniversary of the death of Archbishop Oscar Arnulfo Romero, this country's leading human rights spokesman before he was assassinated one year ago while saying mass.

Although government officials warned Salvadorans in the capital to stay off the streets to avoid trouble, and troops were stationed outside the city's main cathedral, there was calm throughout the country as small groups of worshipers attended a series of masses here and in other cities.

Despite weeks of rumors and speculation that Farabundo Marti National Liberation Front coalition of guerrillas seeking to overthrow the U.S.-backed junta would use the occasion to launch some spectacular military action, only one guerrilla attack was reported, in the small southeastern town of Tierra Blanca. Similarly, there were no confirmed reports of major armed forces offensives, although, as usual, several dozen bodies were reported found around the country at the end of curfew this morning.

Armed forces and diplomatic sources labeled the truce, which apparently was communicated by the guerrilla leadership to its troops through a shortwave radio broadcast last night, a leftist propaganda ploy designed both to take advantage of Romero's high standing in popular Salvadoran memory and to cover up the fact that the left's own military options at the moment are considered extremely limited.

A spokesman for Defense Minister Jose Guillermo Garcia claimed that the left "has lost on all other fronts and now all they have left is to play games with the international media." The spokesman said the armed forces were not going "to give the truce call any importance" and noted that "this isn't a war with defined lines. It is impossible to give such an order to all patrols moving through the countryside looking for guerrillas."

Western officials described the truce as something of a minor political coup for the left in that it appealed to the war-weary nation's deep desire for a respite from near constant violence. "This way," one official said, the left "doesn't have to do anything and can make a perfectly appropriate statement" of reverence for a man many Salvadorans consider a marytr.

Romero was a man of humble origin who originally was selected for the job of San Salvador's archbishop in the mid-1970s by the largely conservative Catholic hierarchy in the belief that he was both similarly conservative and malleable. His radicalization and increasing outspokenness against the government, then an authoritarian military regime led by an Army general, began in 1977, when a Catholic priest who worked with the rural poor here was assassinated by what was believed to be a rightist paramilitary death squad.

Dozens of priests, nuns and Catholic layworkers were killed over the next two years, and Romero repeatedly blamed the military government both for tolerating the deaths, in some cases sponsoring them and in general conducting repressive operations against Salvadoran peasants. Althouth he originally supported the civilian-military government that took over here in October 1979, when the killings continued he began to attack that regime with equal fervor.

Observers here said that the main reason for the low turnout at the various religious services today was the fear of possible violence. "The people are terrified," said one Salvadoran in the plaza in front of the Cathedral. "They remember what happened at Romero's funeral," when some 50 people were killed when violence broke out.

Several persons also commented that leading conservatives in the church hierarchy, who long opposed Romero's outspokenness and accused him of siding with the left, wanted to downplay today's commemoration.

Romero's successor, acting Archbishop Arturo Rivera y Damas, was absent attending a conference in West Germany. Despite continued charges of armed forces repression and sponsorship of political terrorism, he has moved steadily toward supporting the junta as the only viable option for peace here.

[The Vatican said today that Pope John Paul II sent a telegram to Salvadoran Catholic Church leaders calling Romero's murder a "sacriligious assassination," the Associated Press reported.]