The Nicaraguan National Guard, during and after last September's civil war, violated Nicaraguan human rights "in a grave, persistent and generalized manner" - including the "indiscriminate bombing" of civilians and "summary and mass execution" of numerous innocent adults, "youths and defenseless children" - the Organization of American States' Human Rights Commission said yesterday.

The OAS report also charged the National Guard with arbitrary imprisonment, extensive use of torture and "generalized repression" against all young men between the ages of 14 and 21."

The commission's findings, the result of a two-week investigation in Nicaragua last month, could lead to the public U.S. condemnation of the government of Nicaraguan President Anastasio Somoza - a move long urged by Somoza's political opponents and avoided by the Carter administration during the Nicaraguan crisis.

In September, following press reports of National Guard atrocities, the United States called for an OAS investigation "as soon as possible."

"If [the allegations] are true," Deputy Secretary of State Warren M. Christopher told the OAS at that time, "they deserve the condemnation of this body."

Despite, and in many ways because of, the long U.S. history of involvement in Nicaragua and close U.S. ties with the 44-year Somoza dynasty, the Carter administration has strived throughout the current crisis to channel U. S. influence and concern through the OAS. It also has stressed repeatedly the role of the rights commission as a guard against abuses laid to governments in Latin America.

Administration spokesmen have described the Nicaraguan situation as one that should be handled as a hemispheric" problem and current U.S. efforts to mediate between Somoza and the opposition have been conducted under the OAS auspices.

Although a rights commission visit to Nicaragua had originally been scheduled for November, it was advanced in response to the U.S. call. The report, which was drafted and approved in record time by the usually slow-moving commission, confirmed already published accounts of massive rights violations.

Somoza had denied the earlier accounts, and informed sources said yesterday that his government submitted a lengthy, but unpublished, rebuttal of the commission report, which was submitted first to Nicaragua.

The report comes at a crucial stage in the mediating process, which began six weeks ago following a cease-fire in battles between the National Guard and civilians led by the Sandinista National Liberation Front guerrillas.

Somoza has rejected a proposal by an opposition coalition of political, business and civic groups calling for his resignation, the establishment of a provisional government and the dismantling of the 7,500-member National Guard. Last week, the opposition rejected Somoza's counterproposal of a national plebiscite to determine support for each faction and the restructuring of a more representative government under his continuing presidency..

The Broad Opposition Front coalition said that no fair and workable plebiscite could be conducted under the Somoza government, and set a deadline of next Tuesday for its withdrawal from the mediation talks. The guerrillas have said fighting will renew when the deadline passes.

As a condition for a possible extension of that deadline, the opposition has asked for a "symbolic" U.S. show of support for their position against Somoza, such as the withdrawal of the U.S. military mission in Nicaragua.

The human rights report could enhance the opposition bargaining position with the mediation team, which includes representatives of Guatemala and the Dominican Republic as well as the United States.

According to the rights reports, the six commission members who visited Nicaragua spoke first with Somoza and then received "a constant line of persons of all economic and social conditions" who denounced the National Guard. The members "investigated a good number of these cases, visiting the places where the events occurred."

During air and heavy artillery attacks on cities controlled by guerrillas and their civilian supporters, the report said, "the National Guard bombing of the population was done indiscriminately and without prior evacuation of civilians . . . consequently resulting in innumerable deaths of persons uninvolved in the conflict."

Among numerous testimonies and cases quoted as that of a woman in the northern city of Chinandega who lost both her husband and 5-year-old daughter in the fighting.

"It was the 14th of September when the planes started to shoot over our houses in the Barrio La Libertad," the testimony reads. "My husband and daughter were there, and we crouched in a corner of our house, crying and thinking that the [bullets] were destroying our little wooden house.

"We decided to leave and take refuge in a safer place, we left by the kitchen, my husband carrying our daughter in his arms. A plane flew very low, it seemed like it was coming directly toward us, and shot some rockets, hitting my daughter in the back and my husband as he carried her.

"Where I looked, I saw only the heart and intestines of my daughter. She was broken in pieces, destroyed."

Her husband, his arms torn off, walked "about 30 paces, blood flowing from everywhere, until he fell dead. He had a wound in his chest: a smoking piece of rocket was imbedded in his leg."

"She was my only daughter," the testimony included. "I had a hard time having her, and I used to dress her for parties and spoil her. I don't know what I am going to do. I going crazy."

The report described civilians burning alive in their houses, after being told over loudspeakers from low-flying planes to stay inside during the bombardment.