Leftist guerrillas have stepped up attacks in the capital, staging bold daylight raids and threatening to fulfill their pledge to bring the 9-year-old war to the cities.

Urban commandos last week hit the national guard headquarters with mortar and rifle fire, killing four soldiers and wounding 38 people. It was the largest attack in the city in seven years."This is only the beginning," the rebels boasted in a broadcast on their clandestine Radio Venceremos. "Prepare yourselves Salvadorans, the hour is near."

The barracks attack capped more than a month of increased activity by the Farbabundo Marti National Liberation Front, a group of five leftist armies battling the U.S.-backed and -supplied government since 1979.

Friday night, rebel commandos of the FMLN set fire to a bank office and destroyed a power station in attacks in two cities, a military communique said.

FMLN guerrillas raided San Miguel city, 80 miles east from San Salvador, where they dynamited and set fire to Agrarian Development Bank main office and blew up three bank's vehicles at sunset.

Minutes later, in a broadcast of the clandestine station Radio Venceremos, the FMLN claimed responsibility for the attack in San Miguel saying: "This is our welcome and response to Colonel Mauricio Ernesto Vargas, (new) commander of the third military zone, to tell him our urban commandos exist not only in San Salvador."

The rebels say they are showing their strength. The U.S. Embassy says the guerrillas are losing in the country and the city attacks are an act of desperation.

The ultra-conservative National Republican Alliance, known as Arena, blames the raids on a "power vacuum" in President Jose Napoleon Duarte's Christian Democratic government.

Duarte has terminal cancer and, for the first time in years, the March elections will include leftist candidates.

U.S. Ambassador William Walker linked renewed rebel activity to the presidential elections and a Nov. 14 meeting of the Organization of American States in San Salvador.

"With the world's, or at least the hemispheric spotlight on them, they are using dramatic gestures to say `we're here,"' the ambassador said.

The guerrillas promised to take the war to the cities early last year, but until recently their attacks were infrequent and small.

However, since Sept. 13 the guerrillas have maintained pressure in rural areas and moved into new regions of this Massachussetts-sized nation of 5 million.

"As of today I know what the war is. Before it had been only on the television," said Isabel Burgos, a resident of Zaragoza, after an Oct. 6 attack on the town 12 miles south of San Salvador.

That same day commandos using grenade launchers staged a rapid-fire morning attack on the Legislative Assembly in San Salvador.