There are signs that President Anastasio Somoza's hold on this county's National Guard - the main bulwark of his family's dictatorial rule for over 40 years - may be under Strain as the long battle with Sandinista guerrillas grinds on.

The government has launched a major effort to recruit youths into the Guard, with the national radio broadcasting urgent appeals for them to "present yourself at the nearest Guard barracks" to sign up.

A prominent Guard coloneal, speaking after assurances that his identity would not be revealed, bitterly denounced Somoza today before some foreign reporters. The officer said of the president, "This guy is so stupid that he has put the whold regime in danger.

"Now some of us higher officers are in an impossible position. To save our own lives, we have to kill our own people. We know everyone sympathizes with the rebels," he said.

The colonel said he knew of no organized dissident group within the U.S.-trained force but he speculated that "plenty" of his colleagues shared his views. Most high-ranking Guard officers who have spoken with the foreign press have defended their actions as part of a global struggle against aggressive communism.

"It is we who are in the real danger, not the boss," the colonel said today. "He has his millions and his airplanes ready to flee at any moment. But we, the leaders of the Guard, we are the ones who will have to face the firing squad."

If the Marxists within the rebel movement gain power, he said, they "will surely execute us."

The Guard has been solidly behind the Somoza dynasty since Anastasio Somoza Garcia, the current president's father, took control of the force in 1933 and used it to propel himself into the presidency in 1937.

The Somozas are widely reported to have rewarded Guard officers with opportunities to share in widespread corruption.Many officers have accumulated considerable fortunes which they stand to lose if Somoza falls.

By and large the enlisted soldiers are uneducated and motivated by a desire for steady employment and a hatred of communism.

The drive for Guard recruits reportedly has been answered by hundreds of youths. They are said to be given about three days of training and a U.S.-made M16 and then assigned to a unit.

In recent months, the clandestine Radio Sandino has been broadcasting appeals to the 10,000 guardsmen to defect. A Guard lieutenant who deserted on the southern front earlier this month has been held up by the rebels as an example of their good intentions toward those who defect.

In another isolated incident, a Guard pilot flew his Cessna to Costa Rica two weeks ago rather than carry out orders to bomb civilian targets.

There was little change reported today in the military situation which finds the Guard and rebels locked in several battles for control of the country's cities. Here in the capital there was a noticeable slackening in the number of aerial strikes against guerrillas who have taken over many slum neighborhoods.

Following the report from one news agency today that Somoza had agreed to resign his office, his personal aide, Max Kelly, said he was "authorized to deny categorically" that Somoza had any plan to do so.

The Organization of American States has called for Somoza to step aside. There have been reports circulating here that he had indicated a willingness to hand power over to a junta Quintana on the condition that the group bar Marxists from the government and guarantee Somoza's extensive business interests in Nicaragua. CAPTION: Picture, ANASTASIO SOMOZA . . . fate linked to his Guard