At least 14 young men were killed last Friday afternoon on a two-block stretch of Santiago Arguello Avenue here. All of them, according to family members and neighbors, were executed by submachine gun at point-blank range by the Nicaraguan National Guard and all of them begged for mercy, some on their knees.

The eyewitnesses' story of the executions is supported by physical evidence on the scene and by countless similar reports, primarily here in Leon, of National Guard atrocities during nearly four weeks of civil war.

The widespread stories, which include tales of indiscriminate and often apparently accidental close-range shooting of women and children, attest to the ferocity of a war that has made bitter enemies of civilians and those in uniform. The mutual hatred in unlikely to be forgotten soon.

For the National Guard, which ostensibly believes it is saving the country from an imminent guerrilla-led communist threat, every Nicaraguan youth has become a potential terrorist, and every closed door a potential hideout.

In the case of the 14 men gunned down on Santiago Arguello Avenue, the families insist they had not taken part in the fighting. But all were between the ages of 18 and 25, as were most of the rebels who held the town last week.

"They took the first three boys across the street, to that wall," said Adela Alvarez, 38j, pointing out her living room door, "with their hands over their heads and shot them." As two other young men and her son, Carlos, 18, were walking out the door with their hands up, Alvarez said, soldiers in the street machine-gunned their faces and chests. "He was crying, 'Don't kill me, don't kill me.'"

Another son, 13, was hit in the leg as one of the women pulled him back inside the door, she said.

Alvarez is not the real family name. Although the woman insisted on using her true name, it and others here have been changed to protect against reprisals.

Alvarez pointed to the bullet holes in her walls and in the concrete front stoop.

"Right here, this is where they killed my son," she said. Across the street, where she and neighbors said the first three young men had been executed, a concrete block wall was pocked with bullet holes and spattered with blood that was beginning to wash away in the heavy rain.

While low-ranking soldiers on the scene admitted that summary executions took place here last week, officers denied it.

President Anastasio Somoza, asked last night by NBC television about alleged indiscriminate killings, replied: "In any civil strife, in any country when lines are not drawn and you have to go after insurgents, some people get hurt without cause." Asked if he ws satisfied with the National Guard performance, he said, "Yes."

Fighting in this second-largest Nicaraguan city began Sept. 11, when Sandinista Liberation Front guerrillas attacked police and National Guard posts.

According to residents, the ensuing week of occupation by guerrillas and young rebels alongside them was tense but relatively calm.

On Thursday afternoon the National Guard entered a barricaded slum on the orthern edge of the city where, according to Red Cross officials and residents, they ordered the residents of a block to come out into the street. Women and children were reportedly marched north, on a nearby highway, toward Chinandega. Husbands and sons over the age of 15 were marched south, toward Managua.

After walking a mile, the 21 men reportedly were stoped beside the highway, ordered to scratch out a shallow grave in the road shoulder, and shot.

It was not until two days later that the Red Cross, informed by the families of the executed men, arrived to find arms and legs sticking up from the grave. They dug up the bodies, partially burned them, and reburied off the highway in a cotton field.

On Monday, NBC correspondent Fred Francis filed a videotape report from beside the mass grave.

Yesterday, as a heavy rain fell on Leon, the grave was exposed and packs of dogs, nuzzling among flowers placed there by relatives, foraged among the carnage.

On the same Thursday that the 21 were killed, the National Guard began an air attack on Leon, supported by what sounded to those on the outside like artillery shelling from a military post to the west of the city. Yesterday, U.S.-made 105-mm howitzer shells were found in the rubbjle of Leon.

The bombardment ended Friday afternoon. It had been especially severe in the Hermita de Dolores neighborhood, near the center of the city, where rebel barricades had been strongest along Santiago Arguello Avenue, the main street.

The one-story tin-roofed houses on the street are made of adobe and connected like Washington town houses. In yesterday's downpour, women and children picked their way along the rubble-strewn avenue, many appealing to their neighbors for shelter and food.

At 4 p.m. Friday, Adela Alvarez wife of a chauffeur and mother of four, said she was huddling with her own family and that of two neighbors whose homes had been destroyed. They were behind a concrete wall in her small living room when four soldiers kicked open the front door.

"They ordered the women and girls to one side of the room, and the men and boys to walk out to the street," Alvarez said. Outside, she said, an armored car was parked. The killing of her son followed.

As he lay on the front step, Alvarez said, one of the soldiers turned to her.

"He said, 'you are pretty. Maybe I'll come back and visit you.' Then he told me to go to the middle of the street, where they had dragged three of the bodies, and to take a watch off one. He told me to wash it and put it on him. I did it, because I was afraid."

Maria Castilo, 27, was also inside the Alvarez home with her 18-month-old daughter. The soldiers ordered her into the street, she said yesterday, and forced her to walk in front of them, carrying her daughter, for 10 blocks with a machine-gun barrel in her back while they went from door to door. When they let her go, Castillo said, she ran to take refuge in a church.

Several doors down, past a burned-out house and across an intersection piled with rubble, was the house where Christina Medrano was hiding that day with her family, including children aged 5 months, 20 months, and 5 years. When the soldiers came in, her husband, a 22-year-old Health Ministry worker, was ordered into a courtyard along with seven other men.

"They told the women and children to stay in the house, and took the men out onto the patio and told them to get down on their knees," Medrano said. "Then they shot them through the head."

Because the residents were afraid to go into the street, Alvarez said, the bodies of th first three men were left there and burned by the Red Cross after the battle was over Saturday morning. She said she took her own son to a local cemetary.

Four of the men killed in the courtyard were buried there by their families. Medrano said there was no room for her husband's body, so she hid it from the Red Cross burning Brigades until Monday, when she buried it herself in a nearby cemetery.