A senior leader of Guatemala's guerrillas says their chances of mounting a successful revolution there have improved since the conservative minister of defense ousted president Efrain Rios Montt a week ago.

Sebastian Aguilar, a member of the national directorate of the Revolutionary Organization of the Armed People, said in an interview here that Gen. Oscar Humberto Mejia Victores' coup, from the rebels' point of view, "has a positive effect."

Aguilar, who said he represents his group here in what he called a "diplomatic" capacity, termed Mejia's announced plans to hold elections "an effort to distract attention" from what he called the government's repressive policies.

Guatemala's rebels have no interest at present in participating in elections, he said. The insurgents are not ready for or interested in negotiations, either, he added.

Aguilar noted that Mejia is known for his "strong attacks and impolitic positions" in dealing with both domestic and international issues.

The bluntness and brutality of previous military governments had earned the most populous country in Central America the position of international pariah. As a result, the Reagan administration, despite its concern about the advance of leftist insurgencies, has been politically unable to support Guatemala with any significant military aid.

The record of the governments they have fought has been one of the rebels' greatest assets.

Aguilar, one of about a dozen members of the rebel group's directorate, argued that the new leader of the military government in Guatemala probably will be incapable of mounting a more ambitious campaign against the insurgency than his predecessor had.

Mejia is likely "to project an even more negative image of the military regime," Aguilar said.

Guatemala's new leader was known before he took power for a well-publicized argument with U.S. Rep. Clarence D. Long (D-Md.) and a series of erratic and contradictory accounts of how his troops killed a Guatemalan anthropologist working for the U.S. Agency for International Development in February.

Although Mejia reportedly has taken steps to abolish the Special Courts that secretly tried and then publicly executed 15 persons in the last year, he was one of their most outspoken defenders as minister of defense.

After Pope John Paul II's criticism of the practice and the pontiff's unsuccessful attempt to obtain clemency for a group of condemned men in March, Mejia was quoted in the Guatemalan media as saying there would be even more executions.

"You can't protect the people with prayers and pardons," he told local reporters.

Referring to the born-again, sometimes almost messianic president he ousted, Aguilar said, "although Rios Montt had many incoherencies and strange elements about him, about some things he was very careful."

The former president's campaign against the four principal Guatemalan guerrilla organizations, of which the Revolutionary Organization of the Armed People is one, was one of the most sophisticated, both militarily and politically, that the region has ever seen.

"It was a pretty integral plan, pretty complete, that included political as well as military aspects," Aguilar said of Rios Montt's often brutal but effective offensive.

While the guerrillas were clearly caught off balance by many of Rios Montt's actions, they say they expect Mejia to return to a more traditional and, they believe, less effective emphasis on a mainly military response to the insurgency.

Aguilar said that such tactics, even if backed up by direct or indirect U.S. support, are not likely to prove effective.

"The Guatemalan revolution has had a long process of maturation," Aguilar said. It traces its roots to an elected reformist government overthrown with U.S. Central Intelligence Agency backing in 1954 and a mutiny by young officers who then formed the core of the first guerrilla movement in 1960.

"We have gotten to know the Army very well," said Aguilar. "We know it is an enemy to be respected."

In the 23 years of on-again, off-again insurgency, tens of thousands of noncombatants have died at the hands of government troops and death squads, in addition to guerrillas and soldiers killed in combat, according to Amnesty International and other human rights groups.

Well-documented accounts of massacres by government forces, especially since 1978, gained the insurgents international sympathy even as they depleted the number of guerrillas and guerrilla supporters.

At the same time, the successive military governments since 1954 have, as Aguilar put it, "decimated" the political center through death-squad attacks on politicians, labor leaders, students and members of the clergy.

The rebel movement was all but eliminated with the help of extensive U.S. backing in the late 1960s, but as early as 1971 some of the first groups were forming again and some new ones were taking shape.

Their combined forces now are estimated at 2,000 to 3,000 fighters.

In January 1982, when the insurgents seemed to be making major headway against the government headed then by Gen. Romeo Lucas Garcia, the four groups issued a joint proclamation as what they called the Guatemalan National Revolutionary Unity.

Although the four groups are basically Marxist--the insignias of two bear the hammer and sickle; one sports the classic portrait of Che Guevara--the Revolutionary Organization of the Armed People has put a strong emphasis on attracting broad support, including that of disillusioned members of the middle class.

The January 1982 manifesto reflects, in very broad terms, this bid for wide backing.

The rebels continue to operate under that banner. But the overthrow of Lucas Garcia in March 1982 and the aggressive political-military campaign the newly installed president Rios Montt then waged against the rebels cost them much of their open support in the countryside, lost them a large number of fighters and pushed their fragile unity to the breaking point.

The forces of the Guerrilla Army of the Poor, reputedly the most radical group, were hardest hit in the remote provinces of Quiche and Huehuetenango, where a number of peasant massacres were carried out and what amounted to Vietnam-style strategic hamlets were established by the government.

In the past several months, however, Rios Montt's government proved unable to meet its economic promises to the peasants in the troubled countryside or to the middle class of the cities.

This summer, particularly, the rebels began to regroup once again and, according to Aguilar, there are now units of his organization and another called the Rebel Armed Forces operating jointly, if sporadically, in Chimaltenango province, less than 70 miles from the capital.

The once-broken urban guerrilla front also has renewed its activity gradually.

While the Guatemalan rebels do not present an immediate threat to the government, Aguilar suggested that their gradual resurgence was a factor in Rios Montt's downfall, as his claims of having beaten the region's oldest insurgency once and for all appeared less and less credible.