Cannes director Thierry Fremaux insisted on Thursday that festival-goers keep their masks on after a flurry of complaints over stars smooching on the red carpet and letting their face coverings slip in the cinemas.
"Masks are the rule, the law, and the guarantee that the festival can reach the finish," Fremaux told reporters.
He was responding to pictures spreading on Twitter of screenings in which many had removed their masks.
Many had also noted that stars including Jessica Chastain and Carla Bruni had happily taken the traditional pecks on the cheek -- officially banned under social distancing rules -- when they appeared on the red carpet for opening night on Tuesday.
The festival is running until July 17 under strict regulations, with festival-goers needing proof of vaccination or tests every 48 hours to enter the main hall.
France's Covid numbers had been falling steadily, but have seen an uptick in infections as the more contagious Delta variant takes hold, with the government due to meet Monday to discuss new measures.