El Salvador's left-wing guerrillas said that they are shifting their war south from their traditional strongholds close to the Honduran border toward this capital and other, previously less affected areas following yesterday's raid on a communications installation overlooking San Salvador.

Reports broadcast last night by both of the insurgents' clandestine radio stations appeared to confirm recent suggestions by military sources and some U.S. officials that the Farabundo Marti National Liberation Front was moving some of its guerrillas south and west from its longtime strongholds in northern Chalatenango and Morazan provinces.

During the past year, the armed forces have stepped up pressure on these provinces, seeking to cut their supply routes and regularly sending troops through them on sweeps. The rebels' broadcasts challenged a recent suggestion by the armed forces chief of staff, Gen. Adolfo Blandon, that the rebels were bottled up in "pockets" along the border.

"The [front] has launched its border 'pockets' at the enemy's rear guard," Radio Farabundo Marti said. "Gen. Blandon will have to take back his words."

The radio also said that the raid "reaffirms our position regarding the electoral masquerade mounted by the regime and the Yankees," referring to national legislative and municipal elections on March 31.

Guerrilla activity has fallen in northeastern Morazan and neighboring San Miguel provinces in the past year, according to residents interviewed recently. In contrast, guerrilla activity has picked up in Usulutan province to the south of there, according to military sources and Salvadoran press reports.

In the west, the insurgents have shifted some forces and rear-guard operations from Chalatenango south and west to Santa Ana province, guerrilla spokesmen and press reports said.

Guerrillas of the Popular Liberation Forces, based in Chalatenango, staged the attack on the communications installation perched on the narrow, highest peak of the extinct San Salvador Volcano overlooking downtown six miles to the east.

The guerrillas crept up steep, heavily forested slopes and fired mortars at concrete pillboxes on several sides of the peak, defended by National Guardsmen, according to armed forces and insurgent radio broadcastss.

The military said it suffered seven dead, two missing and four wounded and said it had no figures on guerrilla casualties. One of the rebels' radio stations claimed that the insurgents killed or wounded 60 defenders, but the other station did not mention casualties. Reporters who visited the installation late yesterday afternoon saw one dead guerrilla, and other reporters saw the bodies of the seven soldiers removed by helicopters.

The guerrillas heavily damaged four pillboxes of the eight that a government officer said were on the perimeter. The rebels overran one end of the fenced-in center and blew up a telecommunications equipment supply building and several of the troops' huts, the armed forces and insurgents said.

They also damaged the bases of two commercial television towers but only briefly knocked them off the air. The guardsmen drove off the guerrillas in four hours of fighting.