President Anastasio Somoza has categorically rejected a resolution of the Organization of America States calling for his resignation and ordered his National Guard commanders to find new recruits, maintain unity and continue fighting.

Late saturday night, on a monitored police band radio hookup with commanding officers battling Sandinista guerillas thorughout the country, Somoza said the OAS resolution "encourages us to continue struggling."

Mass withdrawal of international support for his government "means nothing more than that we will have to fight with only our own resources," Somoza told the commanders.

"It doesn't matter to me what some countries say who don't know the reality of Nicaragua," he said.

A 17-nation OAS majority voted Saturday for the "immediate and definite replacement of the Somoza regime" and the installation of an opposition government.

In a separate 10-minute televised speech to the nation this afternoon, Somoza did not even mention the subject of resignation. The OAS resolution, he said, "is a clear intent to violate the sovereignty of the Nicaraguan people" but is "not obligatory for member countries."

"It only urges members bilaterally to look for peaceful solutions to the Nicaraguan problem," Somoza said, "As a man open to dialogue . . . I want to say to all Nicaraguans and to the rest of the world . . . that my government is disposed to receive initiatives of the OAS."

As Somoza spoke today, National Guard aircraft prepared to resume bombing a two-square-mile area of eastern Managua held by the guerillas.

Although the government has bombed rural areas in the south to combat a Sandinista invasion from neighboring Costa Rica, Saturday was the first evidence of National Guard use of bombs, rather than small fragmentation rockets launched from the wings of aircraft, in urban areas.

As residents of middle-class suburbs in the hills around Managua watched in awe, government helicopters Saturday hovered at several thousand feet over the low-lying slum areas of guerrilla concentrations while large, clearly visible bombs were rolled out of their hatches.

After what seemed interminable, slow motion drops to earth, the bombs exploded with a vibration that shook windows three miles away.

Guard patrols today prohibited all entry into the area, including that of Red Cross workers seeking to aid the wounded. At midafternoon, the government radio repeated Saturday's call for civilian evacuation of the eastern neighborhoods of Meneses, Maria Auxiliadora, Santa Rosa, Blandon, El Eden, Luis Somoza, Bello Horizonte and Nicarao before resuming the bombing.

Informed sources reported that a Sandinista plane, one of several small aircraft the guerillas are believed to have, dropped supplies early today to rebels barricaded inside the area. As the plane flew over the balcked-out city at approximately 2:30 a.m., the night shook for 10 minutes with deafening but apparently unsuccessful government fire at the plane from the ground.

Meanwhile, after 25 days of sporadic fighting, the city of Masaya, less than 20 miles south of Managua, fell to Sandinista forces Saturday night as National Guard troops evacuated the garrison there. Today, Masaya was calm, but residents were expecting a major effort to rout the guerillas either by air bombardment or by artillery fired from a National Guard fort on a hill overlooking the city.

Masaya has seen some of the harshest fighting of any city in Nicaragua during the 15 months of the revolt. Guardsmen in the Masaya garrison withstood dozens of attacks, and as recently as last week seemed to have weathered the assault. Against this background the sight of the barracks in ruins was a great morale boost to the Sandinistas.

It is the third city to have fallen to the guerrillas. Both Diriamba and Leon are in rebel hands and today more fighting was reported in several northern cities. But on the southern front, where the rebels had originally hoped to establish a "liberated zone" from which to appeal for international support, Guard troops have stymied Sandinista advances and thus frustrated the hope of as many as 1,000 rebels believed to be in the border area.

While Somoza's speech to the nation today was presidential in tone, Saturday night's address to his commanders, following an hours-long Cabinet meeting after the OAS vote, was that of a commander-in-chief with his back to the wall.

Somoza began by reading the entire text of the OAS resolution and asking the commanders, one by one, to acknowledge they had heard it. "I am at your command, my general," said one.

"I am willing to continue the fight and defeat the enemy," Somoza told them. "I want to notify you that this is my decision, and I asked you to acknowledge it." Again, each commander responded affirmatively.

"Okay, boys," he told them. "We know the reality. We must continue forward. Keep on recruiting friends, because we are going to strengthen the National Guard. Keep on fighting."

In radio announcements throughout today, the government called for Nicaraguans to "present yourselves to the military commanders [and] ask to join the Army. The country needs you. Let's join to defend it."

Not all soldiers are heeding those appeals, however. Two patrols boats with a total of approximately 80 Nicaraguan merchant seamen deserted the government Saturday and landed in Panama.

Also Saturday, a commercial airplane from the national airline Lancia-owned by Somoza-flew past its scheduled stop in Managua on its daily run from Miami. The pilot continued to San Jose, Costa Rica, where he announced his intention not to return to Nicaragua. He was joined by the copilot and navigator.

Meanwhile, with Guardsmen who usually patrol the streets of Nicaraguan cities completely absorbed in the civil war, robberies and street crime have been become commonplace. Although most Nicaraguans are remaining inside their homes or in refugee centers to avoid the fighting, foreign journalist here are forced to cover the story in the streets.

On Friday, a correspondent for the French newspaper Le Monde was stopped by a gunman in civilian clothes on the Managua-Masaya highway. The gunman took his luggage, working notes, money and car.

Today, three reporters at the Hotel Ticomo, where many journalists have moved after evacuating the downtown Intercontinental, were robbed of their money and watches. The robbers, six young civilians in white masks, identified themselves as Sandinistas. CAPTION: Picture 1, A National Guardsman reloads his gun during battle in a suburb of Managua. UPI; Picture 2, Sherman tank with 105mm cannon goes into action against rebels in Managua. UPI; National Guardsmen are ready for combat while occupying a house in Managua.