Thieves stole 140 invaluable Mayan, Aztec and other Mexican Indian artifacts from this city's world-famous National Museum of Anthropology on Christmas Eve in the biggest heist ever of pre-Columbian art objects, museum officials said today.

One or more thieves pried open seven glass display cases in three exhibition halls and grabbed several of the museum's best-known gold, jade, turquoise and obsidian objects. Mexican prosecutors detained and were interrogating the nine police guards assigned to the museum on the night of the theft.

Museum officials estimated the total value of the stolen goods at "many millions of dollars." Both Mexican and U.S. experts emphasized, however, that it was difficult to set a specific worth because many of the artifacts are unique.

"They robbed a piece of our history. How can we put a price on it?" Felipe Solis, a museum curator, said at a late-afternoon news conference. He said one of the stolen objects, an Aztec obsidian vessel in the form of a monkey, was estimated by the museum to be worth 10 billion pesos, or $27 million at the official exchange rate, but he stressed this was an arbitrary figure that did not reflect values on the black market for artwork.

Solis and several other experts said the robbery was one of the biggest museum thefts of all time. None of the artifacts was insured, as the museum insures objects only when it allows them to be taken outside the building.

The museum said in a statement that it believed the thieves were professionals who planned to try to sell the artifacts abroad. But several U.S. experts said the thieves had taken such well-known objects that they would find it difficult to unload them.

"It's like trying to fence the 'Mona Lisa,' " Robert Childs, director of collections at the Los Angeles Museum of Cultural History, said in a telephone interview. Any unscrupulous collector who bought the stolen objects would have to "hoard them, because that's all you can do with them," he said.

A former New York dealer in pre-Columbian artifacts expressed fears for the fate of the objects when the thieves discovered how difficult it was to sell them. "No reputable dealer would dare get even his fingerprints on them," the former dealer, who asked to remain anonymous, said. "The tragedy of this is that [the thieves] may become scared and destroy them."

The stolen objects are small and easy to transport, as the largest is only 10 inches in diameter. The best-known pieces included a jade mask from the Mayan ruins of Palenque in southern Mexico, a mask of the "bat god" of the Zapotec culture and the obsidian monkey vessel.

The artifacts were made by the Indian cultures that flourished in Mexico from the 6th century A.D. until the Spanish conquest in the 1500s. The thieves grabbed almost all the museum's holdings of artifacts from two major Mayan finds: a king's tomb at Palenque and a "sacred pool" at the well-known ruins of Chiche'n Itza' in the Yucatan.

The theft was not discovered until the day shift of guards arrived at the museum Christmas morning at 8, museum authorities said. It was unclear how many thieves participated in the heist, how they entered the museum or how long they were there, the officials said.

The eight unarmed police guards and their captain assigned to the 12-hour shift on Christmas Eve were supposed to visit each room in the sprawling, modern museum at least once every hour, according to guards who were at the museum today.

"They [the guards] are being interrogated. None have been charged with anything," Antonio Camargo, spokesman for the National Institute of Anthropology and History, said. The institute operates the museum, which houses one of the world's leading anthropological collections, and has its offices there.

The thieves apparently knew exactly what they were after. They left dozens of display cases untouched and cleaned out the ones containing the artifacts that were the most valuable and the easiest to transport.

"The robbery didn't happen by chance. They stole the best ones [objects] from each culture," Camargo said.

Museum officials showed journalists the empty display cases today, although the museum was closed until Friday while investigators took fingerprints and examined the grounds. The thieves pried off the wooden moldings at the bottom of several cases and simply removed the panes of glass. The museum does not have an electronic alarm system, although officials said they had beefed up security only two months ago in ways they declined to discuss.

"The security system was more than good. It was one of the best," Enrique Florescano, director of the institute, said at the news conference. He and other officials said the building had not suffered a major theft previously in its 21-year history.

But Florescano noted: "Apparently, objectively, the system didn't work. We're going to check it."

"We've never had problems before. I believed that the system functioned well, but times change," curator Solis said.

The museum described the crime as the latest in a series of major art thefts in Europe, the United States, Africa and Asia.

"Such important and valuable objects . . . were taken by criminal hands that, surely, are linked with the world traffic in cultural objects that constantly threatens all of the world's museums," a museum statement said.

"Unfortunately, the removal of pieces that the museum has just suffered joins the wave of robberies that have affected the principal museums of the world in recent years," the statement said.

The most recent major theft of pre-Columbian artifacts took place over Labor Day weekend in 1982 when thieves stole 25 Mayan jade ornaments valued at approximately $500,000 from the Museum of Natural History in New York City. Four of the ornaments were recovered later in Brussels when a dealer became suspicious and the holder of the objects abandoned them. That robbery remains unsolved.