Some 12,000 angry peasants, many of them armed with machetes, have blocked the access roads to Mexico's most important oil field and petrochemical works to protest pollution originating in the complex.

The peasants charged that chemical fallout and oil spills are killing their cattle and ruining their crops. They are demanding that Mexico's state oil company, Pemex, pay millions of dollars in compensation before the peasants will clear the roads.

The blockade went up at noon yesterday, stopping the flow of traffic of people and goods to the crucial Reforma oil fields in the southeastern Mexican state of Tabasco and to the nearby Cactus chemical complex. The complex has one of the world's largest gas sweetners and produces 2 billion cubic feet of natural gas a day, supplying Mexico with two-thirds of its own consumption and exporting large quantities to the United States.

Hundreds of vehicles carrying supplies and workmen have been backed up on the roads to the oil wells and pumping stations that lie scattered among the fields in the area's tropical lowlands.

Pemex has called in the Army to help protect the installation and said that, if necessary, it will start using helicopters to fly in crucial supplies and personnel.

The battle between the peasants and the oil men here has been going on since 1976 when Mexico decided to step up oil production. Heavy equipment invaded the region, farmland was expropriated, and oil started dirtying the lagoons.

The peasants, who say the land they worked was once Mexico's most fertile, charge that the pastures, rivers and brooks where their cattle roam and drink, have been heavily contaminated by acid from the chemical works and oil spills. They say the cattle are dying and their harvests have been drastically reduced.

A spokesman for the protesters said that of the 24 to 30 million pounds of cocoa beans the area used to produce have been reduced to less than 10,000 pounds.

"The zinc rooftops and the steel wiring around the fields gets corroded by the acid in six months," said Jose Luis Solis in a telephone interview.

Solis, who is the municipal secretary of Pichucaco, a village in the heart of the oil district, added that "if that's the way the acid affects metal, you can imagine what it's doing to the people here. Pemex manipulates and lies to us. They think we are stupid. But the people from all the villages here are supporting the blockade."

Talks began today in the state capital of Villahermosa between Pemex officials and the state govenor, who represents the peasants and has often been sharply critical of the oil company.

In a communique, Pemmex said it has paid all outstanding claims for damages and, moreover, it has an office where people can follow proper procedures to file their claims.

In the past few years, peasants have staged demonstrations to protest slow payment for expropriated land, and fishermen have marched to Pemex offices showing buckets of dead fish. But the current blockade, with its interference with production, is the first protest of its kind on such a large scale.