President Anastasio Somoza declared yesterday that the National Guard, is calling up reservists and recruiting civilian government employes to augment its 7,500 troops fighting guerrillas in Nicaragua's civil war.

Somoza told a news conference yesterday that the latest assault on his government was the Nicaraguan guerrilla equivalent of the 1968 Tet offensive in Vietnam.

Referring to the coordinated attack starting Saturday on police posts in five cities, he called it "the bloodiest, most massive" assault here in 50 years. The Somoza family has ruled Nicaragua for 40 of those years.

While Somoza put Guard casualties in the past three days at 30 killed and 75 wounded, he said he had no figures on civilian casualties. Some opposition sources have put the number in the hundreds.

Nicaraguans fleeing Masaya, the country's fourth largest city, some 25 miles south of here, said the streets were filled with bodies of both civilians and National Guard troops. All entrances are blocked by government soldiers, and no one has been permitted to enter since troops backed by helicopter gunships intensified their assault on the rebels Monday.

Ismael Reyes, president of the Nicaraguan Council of Red Crosses, said yesterday that ambulances have been denied entry to Masaya and Esteli, in the north, for the past two days. The government declared martial law in both places Monday.

Somoza said he spoke with the Red Cross yesterday and gave orders to permit entry to the city. "But there's still fighting there," he said, and Red Cross safety could not be guaranteed. Somoza also said that reporters would be permitted to enter Masaya today.

Of the five cities attacked Saturday, Somoza said that Managua was completely under government control. While the city appeared calm, the past 10 days have shown a marked increase in the number of businesses and shops whose doors are closed in cooperation with a nationwide strike called two and a half weeks ago by anti-Somoza opposition groups.

Outside of Managua, however, there is little doubt that Nicaragua is now engaged in a civil war, complete with military operations reports and an end to the freedom given reporters to observe what a few weeks ago seemed barely a series of skirmishes.

Masaya, Somoza said, was recaptured by the National Guard, the country's combined army, navy and air force, at noon yesterday, although shots were still heard from outside the city. Refugees said all water, electricity and telephones were off, and many buildings had been burned. Food shortages, they said, were severe.

Somoza said that Leon, to the northwest, had been secured by the guard within 24 hours of the Saturday attack, and that Chinandega, two hours north from Managua near the El Salvador border, was still partially barricaded by rebels.

"We are in the process" of cleaning out Esteli, 99 miles north of here, Somoza said.

Reyes, of the Red Cross, described the situation in Esteli as "worsethan Matagalpa," a north central city where violent civilian rebellion first broke out nearly three weeks ago.

Despite the continued report of new battles, Somoza appeared to regard the warfare as isolated and temporary insurrections.

"My latest analysis, of the current stage of rebellion," Somoza said, "is that this operation is going to last about a week. In my opinion, this parallel to the Vietnam Tet offensive has been defeated. We are now in the process of cleaning out the diehards."

Somoza repeated his pledge that, despite internal and foreign pressure, he will not resign.

Suggesting that his government still has U.S. support, Somoza pointed out that the economic aid we are receiving from the United States has not been stopped."

Several weeks ago, despite pressure from anti-Somoza factions in the U.S. government, the United States decided to approve $12 million in economic assistance to Nicaragua on grounds that Somoza has been trying to expand civil and political liberities here.

While Somoza said he did not believe that the guerrillas were being opened aided by any other government, he said that the National Guard yesterday stopped 50 guerrillas entering Nicaragua north from neighboring Costa Rica, and pushed them two or three miles back over the Costa Rican border.

The Costa Rican government declared that a Nicaraguan plan dropped a bomb four miles inside Costa Rican territory, injuring one person. At his press conference Somoza made no mention of aircraft being involved in the clash.

A Costa Rican peace initiative, aimed at mediation in Nicaragua, has failed because of Costa Rica's lack of objectivity, Somoza said.

The initiative took the form of a call of a meeting of the heads of state of the five central American nations - Guatemala, Honduras, El Savador, Nicaragua and Costa Rica - who are parties to a mutual defense pact. While Somoza said he would meet with the others, informed Central American observers said the initiative has failed to get off the ground because of the reluctance of the other Central American governments to become involved.

Asked if he considered Nicaragua to be in crisis, Somoza answered in the terse style that has become a trademark. "Yes." Do you feel, he was asked, that if the crisis continued it may do irreparable harm to the country. "No." Perhaps economic harm? "No."

Since Sandinista guerrillas - named after a 1930s national guard general who fought the current president's father - launched a new anti-Somoza offensive last year, the country's economic and political situation has progressively worsened.

The opposition, which includes all segments of society except the Guard, has become more radicalized.

During the past three weeks, nearly all cities outside of Managua, and parts of Managua itself, have been paralyzed by a strike called by the non-guerrilla opposition.

Guerrillas have hit again and again, usually supported by local citizens to the point where it has become impossible to believe that all the fighting is the work of approximately 250 hardcore guerrilla terrorists, as Somoza said yesterday.

Asked yesterday why the thousands-strong National Guard needs civilian reinforcements to fight such a small number of guerrillas, Somoza momentarily faltered.

Each rebel sniper, he said, "sits in a window" from which he has a wide range of fire. "In order to neutralize him," Somoza said, "you need three or four times as many" troops. Somoza said he did not know how many civilians had been recruited or how many will be needed.

Asked about the reportedly large number of civilian deaths, Somoza called the situation "very bloody, very difficult . . . It is unfortunate that these guerrillas hide in civilian houses," he said. "We are very sorry," for civilian deaths, "but this is one of the bloody things about" guerrillas. CAPTION:

Picture, Nicaraguan National Guard forces advance along a street of Masaya, now said to have been retaken from rebels., AP