Nicaragua asked yesterday for a meeting here under the hemisphere's mutual security treaty to consider an alleged guerrilla invasion from neighboring Costa Rica. Fighting was reported on the Nicaraguan side of the border as well as in the northern city of Leon.

Nicaraguan Ambassador Guillermo Sevilla Sacasa told a meeting of the Organization of American States, which administers the security treaty, that "this new invasion" is the latest provocation of "international communism."

Costa Rica denied any invasion. A meeting under the treaty was tentatively scheduled for Monday.

On two occasions since Nicaragua's abortive civil war last September, Costa Rica has invoked the pact - known as the Rio Treaty - to denounce incursions by the National Guard of President Anastasio Somoza.

Last November, the OAS sent a team of observers to the border, two of whom were caught in the crossfire Tuesday as the National Guard battled Sandinista guerrillas.

According to news service reports from Managua, the two OAS observers were pinned down by guerrilla fire outside the town of Rivas - spending six hours on the floor of their car, which was hit several times. They eventually were evacuated by Nicaraguan helicopter.

A Sandinista spokesman in San Jose, Costa Rica, said a guerrilla column attacked Nicaraguan Guardsmen on the banks of the San Juan River that forms the border an killed 22. The Guard denied those losses but confirmed the death of Capt. Manuel Sacasa, assigned as coordinator with the OAS observers.

In a report to yesterday's OAS meeting, the observer team related that in the two months ending May 16 the border area had been largely calm.

The team headed by Grenada's former ambassador, Fabian Redhead, reported that despite numerous Nicaraguan Complaints of Guerrilla provocations from the Costa Rican side of the border, inspections there showed "no evidence of such activity."

Costa Rica does not deny that guerrillas have camped in its territory and have crossed the sparsely tended border, but it does deny assisting them. As the area's sole democracy, Costa Rica has broken relations with the Somoza dictatorship and sought regional cooperation to bring it down as a threat to Central America.

In yesterday's OAS session, Venezeula said that at Monday's meeting it will bringing up the highly critical report on Nicaragua by the Inter-American Human Rights Commission.

In the fighting yesterday, witnesses in Leon, 55 miles northwest of Managua, said scores of guerrillas were killed in street fighting.

President Somoza's half brother, Gen. Jose Somoza, reportedly led a convoy with a World War II Sherman tank into Rivas, north of the Costa Rican border, to put down guerrillas there.

Sadinista leaflets distributed to news agencies in Managua declared, "The final offensive against the Somoza regime has finally begun."

The Nicaraguan Foreign Ministry, following up its charge Tuesday of 300 Sandinistas having invaded from Costa, Rica, quoted "reliable sources" as saying a Cuban airline had landed in Panama Tuesday carrying 200 men, arms and ammunition.

Nicaragua previously charged that the Panamanian government is arming the Sandinistas who take their name from a rebel who fought U.S. Marines occupying Nicaragua 45 years ago. CAPTION: Map, no caption, By Richard Furno - The Washington Post