They come in private planes, in shrimp boats, or simply trek for days through the jungles of Guatemala. Their tools are power saws, shipping crates and, if necessary, machine guns. They are raiders in search of the lost civilization of the Maya Indians, but they are not archeologists in search of knowledge. They are looters motivated by the high fees paid for Mayan relics by art galleries, museums and private collectors throughout the United States, Western Europe and Japan.

Yesterday, at a press conference at the National Geographic Society, a group of archeologists called on Congress to pass legislation that would help prevent the destruction of the ancient Maya civilization.

Among the scholars present was the magazine's staff archeologist, Dr. George Stuart, who on December 29, 1980, led a team into a concealed cave called Naj Tunich, or "stone house." They discovered hieroglyphics, ancient graffiti and even some erotic drawings on the limestone walls dating from abot 733 to 762 A.D., the height of the Maya Classic period. They also discovered there had been other visitors. Stuart said looters had left a pile of pottery fragments and had pried apart the masonry walls.

Stuart said his findings are not unique, and pointed to other examples of illegal excavation in Guatemala:

Recently a band of looters came across an eight-foot stele, or column, in Peten, the northern panhandle of Guatemala and the most fertile ground for Mayan artifacts. A mine of information disappeared when looters cut the stele vertically, like a piece of cheese, destroying glyphs along its side. one half is in the Cleveland Museum of Art, the other in the Kimball Art Museum in Fort Worth.

Last April on a site of Mayan tombs known as Rio Azul, dating from 417 A.D., Guatemalan inspectors surprised 10 looters. A shoot-out ensued. "Luckily they were not good shooters; they were better diggers," said Dr. Francis Polo Sifontes, general director of Guatemala's Institute of Anthropology and History. The looters escaped unhurt with pieces of jade, pottery and an entire tomb. The job, said Polo, was the work of experts.

Polo called the looting a "murder of history," and said, "This fertile field of study . . . is threatened to be reduced to a formless pile of archeological debris and rubble."

According to Polo, looters for the past 15 years have been stripping clean ancient Maya Indian relics from nearly 1,000 different sites in Guatemala and shipping the prized objects. He said the smugglers often cover the relics with gum resin, as though they were exporting the resin itself. Once they arrive at their destination, the objects are reassembled for sale.

The Maya civilization, which reached its cultural height between 300 A.D. and its mysterious disappearance in 900 A.D., could be lost to scholars if the looting continues, according to Dr. Clemency Coggins, a research associate at Harvard University's Peabody Museum. "Not since the 16th-century conquistadors ravaged these countries in their search for gold have the Maya been so plundered," said Coggins. "Guatemala has become the most serious endangered archelogical area in the Western Hemisphere."

Although trade agreements between the United States and Mexico have slowed the trafficking of the larger steles, there are still no U.S. laws barring the import of smaller objects. Coggins said the best way to prevent further looting is a bill now before the Senate Finance Committee based on Unesco legislation barring commerce in stolen cultural property.

Guatemalan law prohibits the export of archeological artifacts, but Polo said his country cannot prevent thefts on its own. "We'd need the entire Guatemalan army to stop the looters," he said.

"If this keeps up," said National Geographic's Stuart, "in 20 years there will be nothing left."