When more than 60 curators, collectors, art critics and law enforcement officials arrived here yesterday to begin a fourday international symposium on art security, the first thing they did was add a new incident to their caseload.

Their bags were hardly unpacked Sunday when the news poured in of the first major theft in the 110-year history of New York's Metropolitan Museum of Art, where a 2,500-year-old Grecian marble head had been stolen from its pedestal during a routine changing of museum guards.

The theft of the marble head, which depicts a bearded man and is valued at at least $150,000, underscored the reason for the conference, its organizers claimed: that the art world is suffering an unparalleled crime wave.

On Christmas day, a guard at San Francisco's Michael H. De Young Museum discovered that thieves who had entered the building through a skylight had stolen Rembrandt's "Portrait of a Rabbi," a 17th-century Dutch picture valued at about $1 million.

Only two days later, officials at the Art Institute of Chicago found that three French paintings by Paul Cezanne, worth a total of perhaps $3 million, had been taken from their frames in a locked storage room and removed from the museum.

"This is a gloomy subject," observed critic Hilton Kramer at the symposium. "Art is under siege."

"Alarm bells are ringing all over the world," said Donald Mason, who spent 11 years investigating art thefts for the FBI. "Time is not on our side."

Although cases involving museums in Detroit, Washington, Montreal and other American cities were cited yesterday, most participants observed that the situation in Europe is even more alarming.

"How serious is the international problem? I'll tell you," said Mason. "Of the 44,000 works of art stolen in Italy since World War II, half were taken in the last six or seven years. You know what they say. If you want to see Italy's art -- hurry."

FBI special agent Richard D. Schwein said that thefts have been traced to "18-year-old junkie burglars, highly sophisticated thieves who steal works of art to order, and museum curators as well. We hear about unscrupulous collectors who steal what they love, but most art thieves steal art to sell it."

He said that the FBI had paid $20,000 to a man in Buffalo, N.Y. in 1977 for a Rembrandt sketch stolen in Bayonne, France in 1973.

"We had a helluva time proving it was a real Rembrandt," said Schwein. "Some expert from the University of Rochester said it wasn't. Thank God, the Louvre came over and said it was. The thief is doing seven years."

No one knows what percentage of stolen art works are eventually recovered, but an estimate of only 5 percent was cited yesterday.

The most famous and familiar paintings are, of course, more difficult to fence and more easily recovered than are coins, porcelains, small statues, prints and other less expensive works of art. Still, some highly visible pictures appear to have vanished. Eighteen Old Master paintings were stolen at gunpoint from the Montreal Museum of Fine Arts in 1972. All are still missing.

Even in celebrated museum cases, sentences are often light. Bonnie Burnham, in a new study of art theft conducted under the auspices of the International Foundation for Art Research Inc., cites, for example, the case of a former Detroit Institute of Arts handyman. The handyman was accused of stealing more than $400,000 in valuable silver and porcelain, as well as drawings by Correggio and Picasso and a painting by Rubens.He was fined $200 and placed on two years probation in August 1977 after he pleaded guilty to charges of receiving and concealing stolen property.

Theodore Donson, a Yale-educated Manhattan art collector and attorney who was arrested while allegedly attempting to remove Old Master graphics from the Metropolitan Museum's print room, was given five years probation and disbarred. Donson has since written a book on investing in prints and is now an art dealer on Manhattan's 57th Street.

Though private collections are far more vulnerable than the world's great museums, even sophisticated security measures do not seem sufficient to prevent all theft. Goya's portrait of the Duke of Wellington, a painting regarded as a British national treasure, was stolen from London's National Gallery by an unemployed truck driver -- who then demanded ransom -- in 1961.

The jewel-encrusted coronation sword of Charles theTenth has disappeared from the Louvre.

Stolen paintings often are surreptitiously exported for resale in other countries. A Viennese millionaire convicted of heading a ring of art thieves was found to be in possession of pictures stolen from Dubrovnik. A masterwork by Piero della Francesca taken from the Ducal Palace in Urbino, Italy, was later recovered in Switzerland, as was a Kandinsky that had earlier been stolen from Otto Preminger.

The transatlantic traffic is two-way. Of eight pictures stolen in Holland in July, 1976, seven were recovered the following December by the FBI in Los Angeles.

"There no longer seems to be any doubt," writes Burnham, "that the professional criminal milieu is involved in fencing stolen art."

For a variety of reasons, precise statistics on stolen art are unavailable. Many thefts, particularly minor ones, are not reported to the authorities, in part because collectors and museums dread publicity and the likelihood of increased insurance premiums.

There is, in addition, no agreed-on definition of a work of art. The Mexican government now contends that all pre-Columbian artifacts buried in its soil are owned by the state. Collectors of such objects might therefore unknowingly purchase works that had been "stolen" from the government of Mexico.

India claims that ancient bronze and stone carvings, stolen or illegally exported from the subcontinent, are now on display in the Los Angeles County Museum, the Cleveland Museum of Art, the Asia Society Gallery in New York and the British Museum in London, Burnham reports. Cross-claims in this field often prompt lengthy litigation.

Though the problem is becoming increasingly acute, few solutions were suggested. A central index listing missing works of art would be useful, participants agreed. No such index now exists.

Representatives of Scotland Yard, the French Central Office for the Repression of the Theft of Art Objects, the U.S. Customs and the insurance industry will speak to the symposium before it closes Thursday.