Venezuela prepared to send air force fighter bombers and a troop transport to Costa Rica yesterday, apparently to protect that nation against Nicaraguan National Guard troops and aircraft who have crossed the Costa Rican border in pursuit of guerrilla raiders.

A National Guard communique here said that three attacks had been launched into Nicaragua from Costa Rican bases by the guerrilla Sandinista Liberation Front and repelled during the past 48 hours.

The Venezuelan move, following tha government's strong criticism of Nicaraguan President Anastasio Somoza and Venezuelan expressions of concern over the situation here, appeared to be escalating the conflict into an international one.

In neighboring Honduras, Associated Press reported, the defense minister said Central American armies might go to Somoza's aid if the Nicataguan president asks for assistance under the Central American mutual defense pact.

AP added that in Panama, 1,500 persons were said to have enrolled in a voluntary brigade to fight against Somoza's forces if he accepts help from outside.

Guatemalan congressman and ex-foreign minister Alberto Fuentes Mohr, visiting in Washington, said Guatemala's vice president, Francisco Vilagran Kramer, had asked to join him in "denouncing any possible armed intervention, open or undercover, in support of Somoza."

The governments of Honduras, Panama and Guatemala are all military-dominated, while Costa Rice has no army.

Venezulean President Carlos Andres Perez, at the outset of the current Nicaraguan crisis, had called for "benign intervention" by the Organization of American States. Venezuela followed that up with a request last week in Washington for a special meetinf of OAS foreign ministers to offer mediation.

However, despite repeated efforts, the OAS has been unable to come to any agreement, even to call the ministers' meeting. Nicaragua has said such a meeting would revive the monster of interventionism" in Latin America.

The word in Washington yesterday was that OAS Permanent Council would meet again today but would ask only for a fact-finding mission.

The Venezuelan offer of planes came in a cable from Perez of Costa Rican President Rodrigo Carazo requesting permission for them to land in San Jose on a "courtesy visit."

That the planes are intended as a warning to Somoza and a promise of Venezuelan support against a Nicaraguan attack on Costa Rica was made clear by the fact that Perez' Information Ministry sent copies of the cable to Carazo to major international newspapers.

Three days ago guerrillas launched the first raid from Costa Rica, meeting strong resistance from Nicaraguan forces reportedly led by Somoza's son, Anastasio III, a National Guard major who directs an elite special forces unit. The Nicaraguans pursued the guerrillas several miles across the Costa Rican border, reportedly strafing them from small planes belonging to the National Guard.

Costa Rica, since 1948, has had no army or air force of its own, and its security forces consist of a small contingent of lightly armed police.

Meanwhile, Nicaraguan political opposition groups called for a diplomatic and economic boycott of Somoza, whose refusal to resign under intense domestic pressure led to the beginning of an all-out civil rebellion here nearly three weeks ago.

Heavy fighting has continued in four of the largest provincial capitals and numerous other small cities since a coordinated guerrilla attack on National Guard installations was launched last Saturday.

Fighting was heaviest yesterday in Leon, Nicaragua's second largest city, where guerrillas joined by armed civilians suffered a heavy air and land attack by reinforced National Guard troops.

Accurate information on the fighting in other areas is difficult to obtain since Somoza Wednesday night imposed nationwide martial law that included censorship of the country's only opposition newspaper. All radio news, other than government communiques, has been prohibited for the past three weeks.