By noon, the heat inside the primitive shelters of black plastic sheeting and waste timber is unbearable. But most of the women and sickly children who live in the camp are off working as day laborers, picking cotton in the fields they covet here in Brazil's most prosperous agricultural state.

They are just some of the 65,000 persons in 42 such camps across Brazil waiting impatiently for fulfillment of the year-old civilian government's promised agrarian reform. Some beneficiaries of redistribution already are on the land, sharpening the conflict with owners who fear confiscation of their acreage because it is not being fully utilized.

Here in the extreme southwest corner of Sao Paulo State, between the Parana and Paranapanema rivers, 2,500 families have camped out for six months in tattered shantytowns, drinking contaminated water from nearby streams.

Pastures of head-high grass stretch away to the undulating horizon, dotted with charred tree trunks of what was once a forest reserve -- and, of late, with sparse herds of beef cattle. "When the cattle arrive, rural workers have to leave," says a field hand.

Only 200 million of Brazil's 1.25 billion acres of arable land are actively farmed and the nation imports $1 billion of food staples. But firm opposition to the redistribution of fallow farms in the developed south, where land values are high, has almost halted the plan.

The encamped families' hopes have been raised by the first tangible effects of reform, for 800,000 acres of land already have been expropriated across the nation -- seemingly as a result of the pressure on authorities that comes with the camps.

Near here, settlers who used similar squatter tactics three years ago, today are harvesting their first crops on a patchwork of fields carved from a former cattle ranch. They receive farm credits, rural extension and schooling from the state government.

This in turn has helped spark a nationwide resistance movement of farm owners determined to alter the government's plans.

The expropriation of 38,000 acres in this area for resettlement of 560 families helped polarize the conflict between cattle ranchers and the landless into something close to a range war, with the government caught between the two sides.

The landowners' Rural Democratic Union, now registered in seven states and with about $250,000 obtained from auctions of thousands of donated beef cattle, was born of the expropriations here in Sao Paulo's southwest, which is a microcosm of the tensions besetting land reform.

The union's founder and moving force is Plinio Junqueira, whose $2.8-million, 7,500-acre cattle ranch was one of the 19 farms expropriated in 1983 under state laws to resettle families.

Junqueira has presented his case to President Sarney and senior military officers as an example of how land reform is becoming "non-capitalist" -- an approach the president rejects.

"We really woke them up to the bad examples of land reform. We have raised a banner across Brazil," said Junqueira.

"They managed to establish the Rural Democratic Union because there's support at the federal level," said Antonio Marco Donaton, regional representative of the state government. He added that the union is growing "because of delays in the reform."

Junqueira presented himself to Brazil's intelligence chief, Gen. Ivan Souza Mendes, as a champion cattle breeder who had even received loans from the Inter-American Development Bank but who had been wiped out by leftist forces.

The state government and its land reform agency say Junqueira was peddling a dangerous illusion, that his deeds are irregular and that half of his farm was expropriated because it occupied the site of a state-owned forest reserve. They also say his run-down pastures were inefficient.

But the farmer is fighting the expropriations in court and is gathering political force among farmers through a nationwide lecture tour. The union says it will elect 40 representatives in this November's polls for a constitutional congress. It plans to herd 10,000 steers into Brasilia for added electoral effect.

"Sao Paulo is the political nerve center for landowners. The owners of 40 percent of Brazil's farmland have their legal residence in the state," said Joao Pedro Stedile of the Landless Workers' Movement.

"We know that leftists are involved in land reform," said the Rural Democratic Union's regional boss, Roosevelt Roque dos Santos. "The key issue is to defend free enterprise and private property -- do you think Nicaragua's example will serve us here?"

No one forgets that one of the catalysts behind the 1964 coup that brought 20 years of military rule here was a land-reform plan. The Brazilian Rural Society, a moderate farmers' lobby, is trying to cool the debate, accepting that both sides must make concessions.

"We admit that idle land should be taxed and that the government has the right of intervention in areas where private property is not playing its role," said its president, Flavio Telles de Menezes. "But to permit the invasion of private property is to take the law into your own hands -- why not let people lynch each other in the streets?"

The recriminations and the state's expropriation of ranches in the region near Presidente Prudente -- a city named for a long forgotten president -- have shown that the need for land reform is not confined to the remote frontier regions.

Agribusiness in Sao Paulo State produces billions in sugar cane and orange juice. Yet there are still more than 10 million acres deemed underused. To meet productivity standards, ranchers are trucking in steers to make up numbers and seeding pastures from airplanes.

Although land titles in the area were twice revoked in the last century and the area declared a state forest in 1942, farmers paid little heed. Experts say half the farmland in Brazil is covered by contested or false title deeds.

Migrant workers from the depressed northeast were attracted south by large landowners who needed them to clear the forests of western Brazil. Once the pastures for beef cattle were prepared, they were ejected. Much of this migrant labor was absorbed in construction of three hydroelectric dam projects nearby. But a debt-led recession starting in 1983 caused mass layoffs and the jobless began their encampments to press for land.

The state government and its electricity utility, which built the dams, recognized a debt to migrant families and expropriated land for them. Now 2,500 more families are demanding land, though only the federal government can grant it.