In the turbulent days of May 1968 in Paris, a group of artists calling itself Atelier Populaire launched a guerrilla operation to turn out thousands of agitprop posters boosting the class struggle.

As student sit-ins and strikes spread, the collective’s secret printing press, hidden inside the prestigious École des Beaux-Arts where the artists also slept, churned out content day and night.

The hastily produced posters, many featuring a clenched fist, were often date-stamped but rarely signed. The message, not the messenger, was of primary importance; the posters were “weapons of the struggle”, not for decoration or display in “bourgeois places of culture” and certainly not for sale – ever. Fifty years on, as France marks the anniversary of May 1968 with a new wave of strikes and student protests, the leftwing idealism of the Atelier Populaire has been trumped by the commerical instincts of the free market. Today the posters are on display in museums and for sale at auctions and galleries across the world with prices topping £2,000.

Currently there are exhibitions at the Musée de Beaux-Arts in Paris, and on sale at the Royal Academy’s London Original Print Fair, the Tate Modern and at Lazinc gallery in Mayfair. The dealer, Gerrish Fine Art, has amassed more than 150 rare and original Atelier Populaire posters, some of which are on sale at the RA fair for between £350 and and £2,250 each.

The agitprop images are said to foreshadow punk art of the 1970s, and Banksy today. Photograph: Atélier Populaire

“These icons of rebellion and civil disorder are the forerunners of today’s thriving street art movement. As monuments to the intense power of the image to bring about change, they represent one of the most potent and striking graphic revolutions in history – fierce symbols of art at the service of insurrection,” it writes.

Art experts now predict a boom in demand for the work that is credited as an inspiration for punk art in the 1970s and Banksy today.

Not everyone is delighted by this belated critical recognition. The French political scientist, Thomas Guénolé, a member of the hard-left France Insoumise (France Unbowed) party is shocked, but not surprised.

“We have a civilisation that cannot accept that there are some things that have no price,” Guénolé told the Observer.

“The Atelier Populaire was vehemently opposed to the posters being sold, clearly declaring it was unacceptable to make money from them. But we are in a society where everything is for sale: workers, culture, museums, public services, our private lives … it’s hardly a surprise that great symbols like this become nothing more than goods to be sold. We see the same with Che Guevara’s image.”

In the spring of 1968, as student protests spread to the Sorbonne and strikes paralysed France, Atelier Populaire produced around 500 different posters reflecting opposition to President Charles de Gaulle, the government, the police, media and capitalism. Printed on newspaper rolls supplied by striking printers, the images were simple but powerful. As the weeks went on they developed a collective style.

Activists worked round the clock to produce the artworks as strikes and protests paralysed Paris. Photograph: Atélier Populaire

Gérard Fromanger, an Atelier artist, said in 1988: “We were having great fun, we lived in the school day and night, it was passionate … and we were the only ones working, which was gratifying. The whole country was on strike except us. We never worked so hard in our lives. We were printing day and night.”

The École was cleared by police in the early hours of 28 June, 1968. They found a few pots of paint and a handful of posters, but not the secret printing press they were seeking.

Georgie Gerrish, of Gerrish Fine Art, who has spent 12 years collecting Atelier Populaire posters, says their message is still relevant. “Everyone is realising how historically important they are. They are from a magic moment full of hope and the idea that if we pulled together we could bring about monumental change. I suppose it does seem strange selling them, but we are now 50 years on, and perhaps the motives for not selling them are not so strong.”

Guénolé, a strong supporter of the ongoing wave of public sector strikes in France, said their sale is “sacrilege”.

He added: “If you consider the values they represent, then selling one of these posters for £2,000 is a misuse of something sacred. And their messages are relevant today, so much so I am using them to make a point about today’s protests.”