Mexican judicial and museum officials said today that the slow response by Mexican authorities to Tuesday night's museum theft of 140 invaluable, pre-Columbian artifacts gave the burglars a head start of at least seven hours before investigators began to hunt them.

Mexican officials also waited a full 24 hours after the robbery, until yesterday morning, to ask the U.S. Customs Service to watch for the Mayan, Aztec and other Indian art objects, according to U.S. Customs officials here and in Washington. That means the burglars had all of Christmas Day to transport their loot into the United States, if they chose, with little risk of detection.

The antiquities may be destined for a wealthy collector's secret horde, a pre-Columbian art expert speculated yesterday. Related story on page F1.

In another apparent lapse, the Customs Service sent an inaccurate description of the missing artifacts to 300 ports of entry, according to an account by Customs spokesman Ed Kittredge. He said in Washington that the telex sent to border posts and airports around the country yesterday after a telephone call from the Mexican Embassy said the objects "are believed to be made of stone or clay," while in fact the overwhelming majority are of gold or jade. He said the Customs officials who wrote the telex "may have been working with very sketchy information."

The Customs Service still did not have a complete inventory of the missing artifacts at midday today, more than 48 hours after the theft, despite U.S. requests, according to Kittredge and a U.S. Customs official here. That seemed remarkable because a museum spokesman handed out dozens of copies of a full, 14-page inventory of the missing artifacts to journalists here yesterday.

"I called the anthropology museum at about noon on Thursday, and they promised me a list. I'm still waiting for it," U.S. Embassy Customs official Joseph Medellin said here today in a telephone interview.

The accounts by Mexican and U.S. officials indicated that the search for several burglars who cleaned out seven glass display cases in Mexico City's internationally known National Museum of Anthropology some time on Christmas Eve got off to a slow and disorganized start. The thieves got away with several of the most famous Mayan and Aztec artifacts, mostly produced between 500 and 1500 A.D., in the biggest heist ever of pre-Colombian art objects.

The initial police investigation suggested that three or more burglars entered the vast, modern museum by means of a basement door that had been left without a padlock, police sources said.

Somebody among the thieves seems to have been an authority on pre-Colombian articles, as the burglars started to take one piece but left it behind, apparently after realizing that it was a replica rather than an original, Mexican officials said.

Several commentaries in today's newspapers were harshly critical of the museum's management for allegedly providing inadequate security. Museum officials said that other institutions also had suffered major burglaries despite having advanced alarm systems, which the museum here lacked.

"It is an investigation that takes time," Enrique Florescano, director of the National Institute of Anthropology and History, which operates the museum, said.

The office of Mexico's federal prosecutor was keeping mum about results of its interrogation of the eight police guards and their captain who were assigned to protect the museum on the 12-hour shift when the theft took place. It was unclear whether the guards were suspected of helping the thieves or only of negligence.

Felipe Flores, a spokesman for the prosecutor's office, and other Mexican officials acknowleged that there was a substantial delay between the time the theft was discovered Christmas morning and the time that the prosecutor's office was notified. That is significant because the prosecutor's office is heading the investigation and was responsible for contacting Mexican and foreign security and customs personnel to keep a lookout for the artifacts. In Latin America, as in the United States, Christmas is a major holiday, and many offices have only skeleton staffs on duty.

The theft was discovered between 8 a.m. and 8:30 a.m. on Christmas morning when the day shift of guards arrived at the museum, according to anthropology institute spokesman Antonio Camargo. The prosecutor's office was informed between 3:30 and 4 p.m. according to spokesman Flores of that office.

Apparently the police guards on the day shift first questioned, then detained those on the night shift. They contacted anthropology institute director Florescano between 11 and 11:30 a.m.

There still was no official word on the number, entry method and escape of the thieves. Museum officials said the burglars were probably professionals who plan to sell the works to private collectors.