Nicaragua leftist guerrillas raided two Nicaraguan owns, adding to the threat to the government of President Angstasio Somoza, who is also challenged by a 12-day-old general strike.

A spokesman for Somoza said querrillas of the Sandinista National Liberation Front crossed the Costa Rican border late Thursday night, then pushed north along the narrow strip of land between Lake Nicaragua and the Pacific Ocean to hit National Guard posts in the towns of Rivas and Granada. At least 14 persons were reported killed and two dozen were injured.

The attacks came as leaders of the broad-based general strike pushed their demands for Somoza's resignation.

Orlando Ruiz Roman, executive manager of the Nicaraguan Chamber of Commerce said the strike has closed or hampered opartions of three-quarters of the nation's businesses. The government says the strike is virtually over, with worker support waning.

Somoza, whose family has ruled Nicarague for four decades, refused opposion demands that he quit.

In Granada, 30 miles southeast of here, 25 to 30 guerrillas wearing the Sandinistas' rod kerchiefs around their necks stole a bus from a baseball team, drove to the main plaza in the city center and fired submachine gums into the National Guard Post, according to reports from the area.

The guerrillas tried to get fire to the Granada post office, fixed bullets into the tires of firetrucks that rushed to the scene, and put up a banner that read: "This square was taken by the Sandinista Front on February 2, 1978."

The guerrillas were said to be using U.S.-made weapons, including 50-caliber machine guns, bazookas, M-1 rifles, grenades, carbines and grenade launchers.

The National Guard, Nicaragua's combined police and army, sent troop reinforcements along with jeeps, tanks and helicopters to both Rivas and Granada.

The political crisis here began Jan. 10, with the assassination of Pedro Joaquin Charnorro, editor and publisher of the newspaper La Prensa and an outspoken critic of the Somoza government. Somoza's critics have blamed him for the killing. The president has denied any part in the killing and ordered an investigation of the murder. Six men have been arrested.

Two days of street protest followed the assassination. Then business and labor leaders joined in the general strike that led to open demands for Somoza's resignation.

(In Washington, Venezuela asked the Inter-American Commission on Human Pights to visit Nicaragua to investigate reports of violations of rights. It was the first time one member of the Organization of American States has unilaterally asked the commission to visit another member nation Venezuelan Ambassador to the OAS Jose Marie Machin said the situation in Nicaragua was "critical."