Gunfire crackled sporadically in a nearby deserted street this morning as Jose Humberto Lopez, an election official, signed the final papers to dispatch another truck full of ballots and soldiers to another rural town.

Preparing for Sunday's elections in the middle of a guerrilla attack on this provincial capital 70 miles east of San Salvador, Lopez hadn't known that another bridge had been blown up during the night or that the roads west were strewn with new ditches, downed telephone poles and high brush barricades, but he just shrugged when he was told.

"I just know that by 5 o'clock we're going to have all these boxes out of here," he said. The 6,000-strong guerrilla force in El Salvador has promised to obstruct the elections they say are a fraud, but the government is fighting just as hard to hold the balloting. Some of the ballots are being delivered by helicopter to areas where roads are cut off.

Lopez said he is a veterinarian, just "one of the people known in town" whom the Central Election Commission pressed into helping out with logistics. "Anyone with the courage to do the work was asked to help," he said. More gunfire, punctuated with the boom of mortar rounds, chattered away in the opposite direction from the earlier shooting as the truck roared off, the soldiers crouched low and ready to fire over the boxes they straddled.

All over El Salvador today, people watched preparations for the voting with varying degrees of boredom, amusement, anger or determination. Guerrilla threats to blow up any bus that dared to take to the highways had left most roads east of the capital empty of any vehicles other than press vans, Army trucks and a few ox carts.

Rebel forces elsewhere in the country stepped up fighting on the eve of the elections, The Associated Press reported from San Salvador. Guerrilla forces were reported in the outskirts of San Miguel, the country's third largest city, in southeastern El Salvador.

The guerrilla's Radio Venceremos said its forces continued to hold the towns of Yoloqiquin and Neanguera, in the northern province of Morazan, a guerrilla stronghold, AP said.

The new ditches across the highway forced all vehicles onto the dirt shoulders where new hand-printed signs put up on trees by the guerrillas were easily visible: "Soldiers! The High Comand and the rich are your worst enemies!" Other mimeographed flyers gave the times and frequencies of upcoming guerrilla radio broadcasts.

The newly blasted bridge looked as though a giant karate chop had split it cleanly across, leaving it folded in a neat V. Guerrillas engaged in heavy fire a mile or so beyond the bridge told reporters they were strung out along the road for the next few miles, harassing two Army trucks trying to get to Usulutan from the east. The guerrillas said the soldiers did not know the bridge was gone.

Apparently they received word later, for the fighting subsided there and picked up closer to the city.

Most areas of the country have had some kind of outbreak this week, just as the guerrillas promised, but fighting has been heaviest for two days now in Usulutan. At least three people have been killed here, including a woman caught in the crossfire, according to local hospital officials.

Maria Flor de Garcia stood in a half-opened doorway looking out on the empty street as the gunfire boomed all around the city. She said it had been worse during the night. "We're all afraid, we haven't been able to shop or anything. Nobody will vote tomorrow," she predicted. Her aunt disagreed. "We'll go to vote--between bullets," she said, and laughed.

Another woman selling bags of toasted bread from a basket on her head meandered down the street with her daughter as shots rattled the windows all around. Asked why she wasn't hiding with everyone else, she said, "If I stay in my house, I don't eat. Nobody will bother me."

As usual in a war, many things functioned normally amid the shooting. The local office of ANTEL, the government telephone company, was open for business and employes cheerfully placed half a dozen calls for reporters within minutes, including one to Washington. Summoned from behind his locked doors, a cafe owner made sandwiches for a dozen people.

International observers accompanied by individual guides fanned out over the countryside to watch the preparations. "The government said we could do anything we wanted, so I took a map and pointed there, there and there," said Robert Wenman, a Conservative member of the Canadian Parliament, at a reception last night. "They said fine, if I didn't mind going to some places by helicopter."

During the fighting in Usulutan today, three photographers, including Gary Cameron of The Washington Post, were caught in a crossfire and took refuge in a service station grease pit, emerging filthy but safe after about 45 minutes. In a firefight near the city of San Vicente, however, a French photographer, Michel Setboun, of SIPA/Black Star, was wounded. He was reported in stable condition in a hospital.