Antigovernment guerrillas, awash in fresh supplies, have begun fighting in larger units in an effort to penetrate Nicaragua's population centers and to intensify attacks on Sandinista troops and economic targets, rebel leaders say.

The guerrillas, who are known in Nicaragua as counterrevolutionaries or contras, have received special training to take cover from artillery and helicopters that the government increasingly is deploying against them. The rebels say they plan to use shoulder-fired antiaircraft missiles against the helicopters, and to obtain an undisclosed type of weapon to "neutralize" Sandinista artillery.

The rebels sent the bulk of their forces, estimated at between 7,000 and 10,000, from base camps in Honduras into Nicaragua in June and July in an effort to stage a sustained offensive for the first time this year. The infiltration was made possible by the rebels' success in opening a pipeline of ammunition and weapons funded by private contributors, according to contra leaders and other sources familiar with their operations.

The rebels' goal is to carry the war away from the isolated mountains, where they have been strongest in the past, and into more populous regions where they can demonstrate their strength for both Nicaraguan and international public opinion. A string of military successes would encourage the U.S. Congress to look favorably on resuming military aid to the contras, as the Reagan administration is expected to propose this autumn, contras said.

Leaders of the Nicaraguan Democratic Force, which is the largest contra group and is known by its Spanish initials FDN, were in a buoyant, almost cocky mood following two successful attacks in early August.

"This war is becoming a bit more tough. We have the supplies to continue fighting at this level," military commander Enrique Bermudez said.

But U.S. and Honduran military analysts cautioned that it is too early to tell whether the rebels would achieve significant advances. The dramatic attacks last month on the Pan-American Highway and on the town of Cuapa in central Nicaragua have not been repeated, these sources noted.

"They're doing much better. The problem is that they're outgunned and outmanned," a U.S. observer said.

These analysts also said that an important test of the Nicaraguan Democratic Force's success would be whether it is able to build popular support inside Nicaragua. The Sandinista government has relocated thousands of peasants forcibly from northern mountains along the Honduran border where the force has had its strongholds. This is considered one reason the contras are now pushing into new regions to seek to expand their base of support.

"The real question is whether the FDN will be able to use military strength for political ends. If they can't break down the Sandinista political structure, then they'll stay in the mountains and just be an irritant," said a source who is in close touch with the force.

In addition, the contras would benefit from a network of sympathizers who could help smuggle supplies. Contra leaders said their guerrillas buy food and some other supplies from the civilian population, but they said their principal "weak spot" was their reliance on air drops for arms, ammunition, boots, uniforms and medicine.

"Our biggest problem is resupply," Bermudez said. The contras are hoping to obtain new aircraft in coming weeks or months to make the supply flights, which originate in Honduras.

Guerrillas have resumed traveling in "task forces" of between 200 and 300 combatants as they did last year, their leaders said. The FDN broke down many of its units into groups of as few as 20 early this year when scarce supplies and Sandinista artillery attacks had forced them onto the defensive, they said. The guerrillas also say they are using their numerous and high-quality field radios to coordinate attacks among several task forces.

In the attack on a stretch of the Pan-American Highway in Esteli province, 2,000 guerrillas from four task forces coordinated an assault at three separate points, rebel leaders said. They used dynamite to blow holes in two bridges and a causeway, and killed at least 29 Sandinista troops in an ambush during their withdrawal. The attack was the most damaging yet in Esteli, a relatively heavily populated province and one of the force's main targets.

"We were operating before in small units. With new training and supplies, we already are operating in larger and better coordinated groups. This attack on the highway marked a new era," said Aristides Sanchez, one of the four members of the force's civilian-military junta.

Interviews with the leaders of the Nicaraguan Democratic Force were granted on the condition that the location be described only as "somewhere in Central America."

Force leaders now say they "lost a year" in their struggle because of last year's cutoff of CIA funding by the U.S. Congress. Contra attacks diminished notably in the first half of this year, and guerrillas reportedly were heading into battles with fewer than 100 rounds of rifle ammunition apiece.

By late spring, however, the force had started to receive boatloads of ammunition and arms in Honduras' Caribbean ports. It took until then for rebel leaders to raise funds and become acquainted with international arms markets, according to several sources.

Now the FDN is said to have approximately 50 million rounds of ammunition. One source said, "They're running out of space to store it." Guerrillas were reported to have at least 200 or 300 rounds apiece at the beginning of their patrols.

The force also obtained additional automatic rifles and other infantry weapons, such as mortars and rocket-propelled grenade launchers. Contra leaders said they had enough materiel to equip 10,000 new guerrillas and are recruiting actively.

As the Sandinistas have stepped up use of Soviet-supplied artillery and MI8 and MI24 (Hind) helicopters, the contras have sought arms to counter these weapons. The FDN has shoulder-fired SA7 ground-to-air missiles, and Sanchez said they had another type of antiaircraft weapon that he declined to identify. Neither weapon has yet been used, however, for reasons that remain unclear.

Contras have been instructed to respond to artillery or air attack by splitting into small groups of three or four guerrillas and seeking cover, the leaders said. Bermudez scoffed at Sandinista claims that the helicopters had caused numerous casualties, saying, "We have not lost a single man dead or wounded from the Hinds."

The new emphasis on striking economic targets such as highways and fuel depots appeared to be a potentially significant shift in contra tactics. The rebels have said they do not wage war on the economy, although they have attacked economic targets such as grain silos, ports and bridges in the past. One of the contras' main targets also has been state farms or state-organized cooperatives, although the rebels argue that the presence of Sandinista militias on these farms makes them military objectives.

The Nicaraguan Democratic Force publicly has disavowed economic sabotage in part to avoid comparisons between its tactics and those of El Salvador's left-wing guerrilla movement. The Salvadoran guerrillas, at the opposite ideological pole, regularly attack economic targets such as electric power lines, saying the nation's economy helps to finance the government's war effort.

The Sandinistas' use of helicopters automatically has made legitimate military targets of all fuel depots and the oil refinery in Managua, rebel leaders said, because fuel is being used in the aircraft for military purposes. Likewise, they said, highways are targets when the Sandinistas use them to transport troops in military vehicles.

"By using this type of equipment, the Sandinistas have opened the doors for us to stage whatever type of attack is necessary to protect our people," Sanchez said. But he stressed that the contras' principal targets were Sandinista troops.

Reports from Nicaragua on Friday said the contras had knocked out electric power in three communities in the north. Rebel spokesman Frank Arana said he had not received confirmation of such an attack.