Members of a closed society of porters at France’s oldest auction house have at last gone on trial, accused of systematically pilfering bric-à-brac and art works including a Chagall painting.

The trial of the cols rouges (red collars, after the uniform) threatens to expose the dark underside of the Paris auction world; 49 members of the private club which has monopolised portering at Drouot for 180 years are accused of stealing tens of thousands of objects. Police recovered 250 tons of pilfered property, ranging from household appliances to antique furniture, diamonds, sketches by Pablo Picasso and paintings by Gustave Courbet and Marc Chagall.

The items, found in a warehouse in the Paris suburbs, also included a trunk of the personal memorabilia collected by the celebrated French mime artist, Marcel Marceau, before his death in 2007. Most of the 49 porters on trial admit the accusations of conspiracy to steal and to handle stolen goods. Some deny the charges. Four auctioneers are accused of knowingly selling stolen property and deny the charges. Drouot has denied any knowledge of the alleged thefts.

Over the next three weeks, the prosecution will allege that a long-standing system of small-scale theft by some porters became a “co-operative of crime”. Objects would vanish when the porters cleared the homes of wealthy people after their deaths. If their heirs complained, the missing item would mysteriously reappear.

One of the porters is alleged to have said this was regarded as harmless, because it amounted to “stealing from the dead”; an open secret in the French auction world for decades. Problems arose when some porters extended the practice from relatively worthless items to valuable works of art. In February 2009, an anonymous caller contacted a French police unit that investigates art thefts. He said that a canvas by the 19th-century artist Gustave Courbet, supposedly lost in 2003, was about to be sold by a red collar for €100,000.

Phone taps are then alleged to have uncovered a network of theft. An investigating magistrate, Ann Bamberger, in her report, spoke of a “near-mafia” system and a practice which had become “habitual, even institutional”.