Celestina Sibrian is a shrewd woman, and it shows as she sits, frail and worn from half a century working her way up from the peasantry, on sacks of sugar in her well-stocked little warehouse, talking about other peasants who say she just threw them off their land.

Or perhaps it is her land.

She saw a newspaper article earlier this month saying the reform that had given them the right last year to claim the little parcels they rented from her was revoked this month by the new rightist-dominated National Constituent Assembly in San Salvador. She made sure her former tenants saw the article, too.

She smiled. "One of them came here very angry and said I was trying to intimidate him," she said. "I said it is not me. It is in the paper."

The assembly's leaders maintain that this sort of incident, repeated thousands of times throughout the country in recent weeks and often with more sinister and coercive overtones, is just a misunderstanding. They say that newly voted measures suspending substantial parts of the U.S.-backed agrarian reform ultimately will help El Salvador, and rightist assembly leader Roberto D'Aubuisson promises that his goal is not to roll back the land-distribution program altogether.

But, according to the Union Comunal Salvadorena, the country's largest peasant organization, more than 3,600 peasants (some estimates are as high as 12,000) have been intimidated, forced or fooled into letting go of tenuously held bits of land since the March 28 elections brought a rightist coalition to power in the new constituent assembly.

Despite legalisms to the contrary, for instance, the controversial phase of the reform intended to give tenant farmers the land they rent up to 17 acres, known here as Decree 207, effectively has been frozen since May 19, the day after the assembly rammed through a measure that exempted virtually all the farmland in the country from its provisions for up to four years.

The assembly is resentfully aware that the U.S. Congress last week made more than $100 million in military aid for the fight against Marxist-led insurgents contingent on the progress of agrarian reform here. President Alvaro Magana, allied to the military officers who were instrumental in pushing through the reforms in the first place, reportedly is considering further moves to clear the air.

Even its strongest backers within the formerly ruling Christian Democratic Party agree that parts of the land reform program should be altered. But the Christian Democrats fought for 20 years for major land redistribution in this tremendously overpopulated little country where, according to a U.S. Embassy report, as few as six rich families once owned as much land as 133,000 small farmers.

For decades, the sometimes paternal but often ruthless system of land tenure had proved resistant to attempts at small-scale change. The newly empowered rightists, Christian Democratic leaders argue, now want to do away with the little that has been done under the guise of reorganization.

The current fight centers not on the big plantations, however, but rather on the murkier area where the reforms give the landless poor claims against property with owners ranging from upwardly mobile peasants such as Sibrian, whose total land holdings amounted to about 25 acres, to what one analyst refers to as "rhinestone oligarchs"--upper middle-class Salvadorans with medium-sized holdings.

Decree 207, or Phase III of the three-stage land reform program, is now the most controversial. Hastily slapped together in April 1980 under pressure from U.S. officials, it contained a clearly impractical provision that made the "indirect exploitation" of all farm land illegal and effectively banned any kind of rental.

On paper this single provision--damaging to the country's crucial cotton and sugar growers--was the assembly's target.

But, meanwhile, the rightists were sending out orders through Agriculture Minister Miguel Myshondt, a member of D'Aubuisson's party who is currently out of the country, that virtually shut the whole program down.

According to Vice Minister of Agriculture Jorge Alberto Pena Solano, a Christian Democrat who studied agrarian economy at the University of Maryland and is an admitted partisan of the reforms, the orders were never put in writing, "because nobody wants to be compromised," but the agency responsible for handing out titles to eligible peasants was told to stop giving them even the receipts that acknowledge their applications.

This account was confirmed by other well-informed, independent sources who asked not to be named.

Much of the ill health of the hastily planned and vastly underfinanced reform program, however, is not the fault of the new assembly leaders.

If major parts of the reform program were not exactly dead when the new government took over here, they were in a coma, with the second and potentially most extensive phase of the reforms announced in 1980 "indefinitely deferred" almost since it was first mentioned. The second phase would have affected farms between 247 and 1,235 acres, including most of those producing coffee, a major export crop.

The fear of the reform's backers is that the new assembly may pull the plug on it altogether, and this could be done in so many ways that it might not be noticed until it is too late.

"We can't talk of a partial or temporary freeze," said Pena Solano of the moves concerning Decree 207. "In our system of laws such a thing is nonexistent. You either do it or you don't do it."

Independent analysts of the reform process tend to agree.

"Any process like this dies if it quits moving ahead," said one agrarian reform expert.

Former president Jose Napoleon Duarte, whose government initiated the reforms, said he personally was opposed to the U.S.-promoted Decree 207 because of its lack of financing, and his party had conceded the need for some exemptions. But the assembly's rightist leadership, he said, "is opposed to the principle" of the agrarian reform program.

The current evictions bring back memories of a time in El Salvador's not-too-distant past that reformers here had promised was gone forever.

When a rural minimum wage was enacted in 1965 to end the virtual serfdom of many families living on large estates, owners evicted those they considered uneconomical under the new law. A United Nations study found that, partly as a result, the percentage of the rural population without access to land rose from 12 percent to 40 percent between 1960 and 1975.

By the time the Christian Democrats were finally in a position to begin implementing reforms--after they joined the government with young military officers intent on asserting the Army's independence from the old economic elite--the strength of antigovernment guerrillas, building on rural discontent, appeared to be growing so quickly that key agrarian reform measures were rushed into effect. Their main emphasis was on their short-term political potential to erode insurgent support, and there was only a haphazard approach to financing and administration.

Duarte said recently that phase one of the reform, which expropriated large farms to be turned over to peasant cooperatives, wound up taking most of the Army, half of a year, 50 percent of all government investment money and half the nation's agricultural credit to even begin. There was nothing left over to start phase two, which was designed to turn medium-sized farms over to the peasants, and it was suspended indefinitely by Duarte even before it had begun.

The third and final part of the reform, Decree 207, affecting the smallest tenant farmers, wound up being launched more or less on a prayer with less than 10 percent of the money needed to carry it out. Implementation did not even begin until nine months after the law went into effect, and not a single peasant has ever received more than a provisional title to the land he rented.

The situation that has evolved is more complex than a simple question of landed oligarchs and starving peasants.

Phase one, which has remained largely untouched by the current controversy, is the major part of the reform directly aimed at the old feudal land system here. Under its provisions, 263 farms larger than 1,235 acres have been expropriated. There are many who believe phase one succeeded in breaking the oligarchy's tradition of unquestioned power, if not taking away their power altogether.

While D'Aubuisson and his supporters recognize some general need for social improvement, they are clearly antagonistic to the specific programs in effect now. They generally favor some variation on the laissez-faire economic system under which several of them became well-to-do.

"I cannot imagine what the Christian Democrats had in mind unless it was to destroy the economic foundation of this country," said one landowner who supports D'Aubuisson.

Before the reforms were announced two years ago, D'Aubuisson, a major cashiered in the reformist 1979 coup, led a passionate opposition to them and tried unsuccessfully in February 1980 to head them off with a coup of his own.

But a year later, in March 1981, while D'Aubuisson was still plotting from underground, he was telling reporters that he would not roll back the reforms and that he just wanted to make them work.

Always outwardly confident that he would come to power, as he finally did to a considerable extent in the election, D'Aubuisson was aware even last year that the Reagan administration had made the reform process the key to further increases in vitally needed aid.

The ex-major still tends to discuss the existing agrarian reform not in terms of its domestic benefits but in terms of U.S. aid.

Referring to the controversial, if moribund, second phase of the reform, D'Aubuisson told a meeting of the American Chamber of Commerce here earlier this month, "If we revoke the law we will antagonize the United States Congress, which will interpret the move as against the reforms. If we do not revoke it, the landowners who are affected will continue to live with uncertainty and be unwilling to invest in their farms."

But while the assembly continues to walk this delicate line, the future of the reforms, Celestina Sibrian's property and the men who thought it was theirs remain in doubt.