The Salvadoran armed forces reported today a massacre of civilians by antigovernment guerrillas at a town north of the capital. The leftist insurgents have long maintained considerable strength and evident cooperation from local peasants in that area.

According to the official communique, the guerrillas attacked the military garrison in the town, Nueva Trinidad, Saturday night. Statements by the armed forces charge that 150 to 200 civilians were slain and one San Salvador newspaper put the figure at more than twice that, based on statements the newspaper said were from survivors of the alleged massacre.

The defenders later fled, by the communique's account, but the guerrillas apparently also withdrew, and Nueva Trinidad is now under Army control.

Foreign television cameramen who visited the scene today, and who have extensive experience in El Salvador, said that on the basis of conversations in the town they believe as many as 50 persons may have died there, including 10 to 15 civilians. Most of the dead that the cameramen could account for were civil guards--paramilitary rural guards ultimately responsible to the central government and the Army.

The cameramen said they actually saw only three bodies, all of soldiers. It was unclear how many soldiers died and how many escaped.

The guerrillas have promised a major increase in their activity and yesterday mounted an assault on the garrison of a provincial capital, Usulutan. Today, fighting appeared to have subsided there.

But the propaganda war that the insurgent leftists and the government are fighting continued in full force.

The reported Nueva Trinidad massacre, which the Army blamed on the guerrillas, follows recent news stories about mass killings of civilians by government forces.

Reporters working for Salvadoran newspapers that unquestioningly support the government were taken to Nueva Trinidad yesterday, and one group of cameramen working for international media were flown to the little town, near the Honduran border, today. Military authorities did not respond to repeated requests by correspondents from The Washington Post and The New York Times for similar treatment.

Carlos Santamaria, a stringer for the American Broadcasting Co. who went to Nueva Trinidad with local reporters yesterday, said he saw 16 bodies. Five appeared to be civilians. Ten were apparently members of the civil guard, and one was believed to have been a guerrilla.

Military personnel showed the reporters a grave, saying it contained 150 bodies. But the reporters said the unopened grave appeared to have a much smaller capacity.

Col. Alfonso Euserbio Coto of the armed forces press committee said in San Salvador that during a visit to the scene yesterday he was unable to count many bodies because of the difficulty of traveling around trails that might be mined.

Both the official communique and local newspaper accounts today relied on quotations from unnamed witnesses who apparently talked as a group. The newspaper El Diario de Hoy carried a headline: "400 Murders by the Terrorists." The other morning paper, La Prensa Grafica, set the figure at 150.

A collective quote in the communique, attributed to "the survivors of the massacre" and published in La Prensa Grafica, said: "It was something hellish! Something we never thought of! They were men like demons, with the desire to kill, they didn't care whether old folks, or women, or children! Those who couldn't flee, who knows what happened to them!"

As for the soldiers in the little garrison there, the armed forces communique, titled "Genocide in Nueva Trinidad," said that the 15 soldiers in the town held out for several hours, but the "numerical superiority of the terrorist hordes decimated the valiant resistance of our heroic soldiers until finally some escaped with their lives."

By all accounts, Nueva Trinidad is a small town. The communique quoted a 1970 census giving its population as 262 and that of the surrounding rural area as 5,749. Journalists said only a few townspeople could be found there yesterday or today.