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Armed police swooped on a Halloween event after a passer-by saw people being bundled into the back of a van by men holding guns and wearing balaclavas.
But the group of 20 men and women were actually participants in a "physical horror'' survival event called Outbreak which was taking place in the village of Albourne, West Sussex, on Sunday, organisers said.
The Halloween experience, created by White Knuckle events, saw ticketholders meet at a secret location in central Brighton where they were taken, by military escort, on to the South Downs.
Participants were told they would be able to crawl, climb and navigate within areas of reduced lighting, and they were likely to get muddy and wet.
Outbreak spokesman David Parker said: "Most of the participants thought the police were part of the event.
"It took a while for them to believe that the officers were actually from a real police force.
"We believe a member of the public walking past the private land where the event was taking place saw one of the scenes where people with replica guns force participants into the back of a truck and called the police.
"Before we knew it, armed police and the helicopter had been despatched.''
Mr Parker said more than 1,000 tickets had been sold for the event, which took place all weekend and involved a cast of 60 actors.
He said the people affected were the last group of the weekend to take part.
Mr Parker said Sussex Police had been made aware that the event was taking place but he understood that they had to take a report of this kind seriously.
He said: "It was definitely a case of crossed wires.
"As soon as they realised the guns were plastic, it all ended very quickly.
"One of our cast members even thought we had set them up and it was all part of the day.''
A Sussex Police spokeswoman confirmed they had been contacted at 8.45pm on Sunday by a member of the public who had seen a number of men wearing balaclavas and carrying guns.
She said the men, who were dressed in black, appeared to be carrying rifles and pointing them down the road.
"Armed officers attended and a number of people in a van were detained,'' she continued.
"Handguns and rifles were seized and examined but were proven to be BB guns being used at a public event.
"The organisers of the event were contacted and given strong words of advice.''
Superintendent Rex Matthews said: "We would actively discourage people from carrying even imitation firearms in any public place.
"The threat level from international terrorism in Britain is currently assessed as severe and there have been a number of terrorist plots thwarted.
"People are being asked to remain vigilant and to report anything that is suspicious.
"At times like this, incidents such as the one on Sunday are clearly going to worry local residents and the public, who were unaware of the intentions of the event organisers.
"We would ask anyone organising such events to be mindful of how their actions could be perceived by others.
"Incidents such as this, where people are dressed in dark clothing at night and holding firearms, are likely to attract the attention of the police and those involved are inadvertently placing themselves at significant risk.
"While we do not in any way intend to stop people enjoying such events or indeed getting fully involved in Halloween celebrations, I would urge people to please use common sense before they get involved in such activities.''