The Brazilian musician Joao Gilberto had a foghorn accompaniment at his concert at the Vienne jazz festival in western France. But the improvisation on Saturday night was not planned by the artist, who abandoned the concert after 40 minutes. It was arranged by protesting arts workers, angered by plans to reduce their unemployment benefits.

France's summer of discontent has already been punctuated by strikes by public workers who halted railway and airline services to protest against the government's pension reforms. Now the country's 650 summer art festivals are threatened, and there are fears for the most prestigious of them all, in Avignon, which opens on 8 July for its annual three-week run of theatre, dance and art.

On Sunday, organisers of the Montpellier dance festival stopped the 14-day event after three days of disruption by the arts workers' unions.

Marseilles followed suit, by cancelling its festival featuring contemporary work. Aix-en-Provence has halted the first performances of its celebrated opera festival.

Other festivals that are facing a boycott by technicians and support staff include two in Brittany, the "Vieilles Charrues" in Carhaix-Plouguer and the "Tombées de la Nuit" in Rennes. In Caen, Normandy, police evicted about 100 protesters who had occupied a theatre for three nights running, while in Paris 50 protesters were thrown out of an exhibition hall in a park.

The protests began after three French unions signed a deal with the main business federation on Friday to reform the arts workers' benefit system. The deal was opposed by two other unions, the CGT and Force Ouvrière, which represent the majority of the arts workers.

The new scheme would reduce the period during which they could draw unemployment benefit and increase the time they would have to work to obtain benefits during their lean periods.

Jean Voirin of the CGT said: "We're angry, an anger largely shared by all professionals, whether they are performers, technicians or workers."

But Jean-Jacques Aillagon, the Culture minister, was adamant that the deal would have "very significant advantages" for arts workers.

Avignon would suffer a disaster if the festival organisers were forced to cancel, because the event brings in €40m (£28m) each year for the city. On Sunday, 150 art workers in Avignon decided to vote every day on whether to continue the strike. Yesterday, the performers were not certain that they could even hold rehearsals.