A fake terrorist attack has panicked guests at the Hôtel du Cap, the five-star luxury hotel frequented by celebrity guests during the Cannes film festival.

The ‘attack’, which was in fact a stunt by a French internet company, happened yesterday, Friday the 13th. Six men in matching helmets and military-style utility vests approached the hotel’s dock via boat, before one of them began marching up the steps towards frightened guests, according to the Hollywood Reporter.

“We were all caught off guard,” said publicist JR Savet, who was eating with friends on the du Cap’s terrace. “And then someone screamed and people jumped out of their chairs and started moving quickly to the swimming pool area. It was pretty scary.”

Savet said that he was shocked by the fact that the men seemed to be dressed as an “Isis-like” militia group.

The ‘attack’ was a stunt by a French internet company. Photograph: Vantagenews.com

Organisers at the Cannes film festival have called in 500 additional security personnel this year as the threat of an attack by ISIS is, following the attacks on Paris and Brussels, heightened.

Last month French security forces staged a mock attack on the film festival’s principal venue, the Palais. “Terrorists” wielding automatic weapons fired blanks at police, before climbing over the “wounded” (volunteers wearing red bibs) and storming the steps of the conference centre.

The du Cap, which sits eight miles east along the Cote d’Azur coast from Cannes, regularly hosts some of the movie industry’s most famous figures during the film festival. On its way to the hotel, the boat reportedly sped past yachts owned by Steven Spielberg and Roman Abramovitch.

Cannes police are investigating the stunt, which a du Cap spokesperson has called “a bad joke – a really bad one”.