Rejecting appeals by Guatemala's new president for them to lay down their arms, the four guerrilla forces operating in the northern part of the country appear to have stepped up their operations against the armed forces and newly organized civilian militias.

The guerrillas, estimated to number between 4,000 and 6,000, seem to be trying to take advantage of the armed forces' uncertainty following the March 23 coup that put Gen. Efrain Rios Montt in power as head of a three-man junta.

Many foreign analysts in the capital, Guatemala City, believe that the removal of president Romeo Lucas Garcia and his brother Gen. Benedicto Lucas Garcia as chief of staff left something of a vacuum of military leadership in the antiguerrilla struggle.

The junta, installed by young officers, has shuffled most military and police commands to shore up their base of support and to prevent any new plots. There have been reports that in at least three instances, units have balked at accepting their new commanders.

Business people in this northern province of Quiche say the confusion in military command has given the guerrillas new freedom to operate in the countryside.

"They are more active than ever," said a businesswoman in Chichicastenango the other day, shaking her head in dismay. "If things keep going like they are, the subversives are going to regroup and be stronger than ever by the end of the year. Then what will happen to us?"

The guerrillas, meanwhile, have shrugged off all entreaties from Rios Montt that they give up their struggle now that the corrupt and repressive Lucas government has been overthrown.

"The recent coup d'etat is a farce and a trick that gives the Army time to breathe," said a communique after the coup issued by the Guerrilla Army of the Poor, the largest of the four guerrilla groups.

Some local commanders such as Col. Paulo Mendez, a burly parachute brigade commander stationed at the sprawling Army base laid out around the edge of this provincial capital's dirt airstrip, still express optimism about the war against the leftist insurgents. Their assessment is not shared, however, by nervous residents of neighboring towns who said guerrilla activity had picked up since the coup.

Col. Mendez's statement that the war is "nicely under control" also was belied by the freshly burned-out hulks of vehicles and the pushed-aside roadblocks of giant pines that a visitor must thread his car through driving here along the perilous road that leads from the capital, 98 miles to the southeast.

The colonel's expansiveness ignores the body found at dawn in the town of Chichicastenango, 11 miles to the south, with its face cleaved by a machete. It also flies in the face of one of his own lieutenant's reports, which interrupted our conversation, of two new ambushes along the winding highway just south of the city.

But Mendez, an expert on psychological war before being sent to head pacification operations in a wild zone known as the Ixil Triangle in the rugged volcanic mountains north of here, is nothing if not upbeat about his war. No amount of contrary evidence that kept interrupting his briefing in a sweltering tent at the edge of the Army base shook him.

The armed forces' nine-month-old campaign of aggressive search-and-destroy tactics against the guerrillas, he said, has isolated the rebels from their traditional civilian base of support. The civilians--either by choice or fear of the Army's often brutal tactics --have fled the hills and congregated as refugees around the towns where the military maintains its garrisons.

"The subversives are no longer a military threat," Mendez said. "They have been forced to break into small groups whose only tactic is to try to terrorize civilians so they won't support the Army."

Just as he delivered the opinion that the guerrillas are no longer operating in large groups, Mendez's briefing was interrupted by a 32-year-old Indian farmer, Rolando Wotsbeli Argueta, who had just been helicoptered in with an Army patrol from his village of Llano Grande along the Rio Negro to the north.

Wotsbeli Argueta, a member of the civilian village defense force that the Army has been organizing in recent months, reported a harrowing tale of an attack on his village by a force of about 300 guerrillas two nights earlier. Two of his brothers and two nephews died in the engagement in which the villagers fought off the guerrillas with .22-caliber rifles and .38-caliber revolvers, the only weapons the Army is willing to entrust them with.

Wotsbeli Argueta, fresh from burying his relatives, spoke of "rivers of blood" found in the morning after the attack, but he said there was no evidence that any of the guerrillas had died.

The Indian farmer's tale confirmed that the guerrillas have decided to take on the civilian defense forces that the Army has been creating as a buffer.

A week ago the guerrillas killed 13 civilian defenders in the small town of Rabinal, in the neighboring province of Baja Verapaz. Here in Quiche, one of the Indian provinces that has been a bastion of guerrilla operations since the mid-1970s, Mendez said that in the past two weeks, 16 civilian defenders have been killed by the guerrillas.

The civil defense force, Mendez said, has taken the brunt of the guerrilla attacks recently because, as he put it, "The subversives know that any attack on an Army patrol means total destruction for them."

The colonel said that in recent weeks, his units have had no casualties, which, given the fact that government forces were being killed at the rate of about 50 a month earlier this year, tends to bear out civilian reports that the Army is not going out against the guerrillas as aggressively as it was before the coup.