A film-maker was arrested by armed police as he was preparing to shoot his latest picture, after two toy cap guns were mistaken for real weapons.

Chris Martin sparked a security scare when neighbours reported that they had spotted guns and claimed a petrol bomb had exploded in a back garden in Bristol. A police helicopter was scrambled and about 20 officers – including an armed response unit – swooped and arrested Martin.

But the weapons turned out to be a harmless toys and the explosion a controlled one carried out in preparation for the film, which tells the story of a young man's struggle in credit-crunched Britain.

Martin, 42, said: "Police cordoned off the whole street and arrested me. I was told to put my hands up and warned that the officers had loaded weapons.

"I was shouting 'they are prop guns, they are prop guns!', but I was handcuffed, put up against a wall and ordered not to move or speak. There must have been around 20 police there in total.

"I think there were some pretty red faces among the armed response unit when they entered my home after arresting me and being met by two cameramen."

Martin, a city councillor, said he had visited a police station to show them the guns and tell them they were going to be used in his film, Cal.

But it went wrong on Saturday when he held a prop day at his house in Montpelier for his crew to go through some stunts.

They had painted two toy guns black and were working with fake bottles filled with liquid that could create a modest explosion.

Police realised the mix-up after 15 minutes and de-arrested Martin. He said: "I don't have any complaint against the police. They behaved perfectly properly and I realise they were doing their job.

"But I am a little concerned about the cost involved because a couple of quick phone calls would have resolved this without going to all this trouble."

Avon and Somerset police confirmed that armed officers and the force helicopter had been sent to the scene.

A spokesman said: "We received reports of two handguns at the property and that a petrol bomb had been set off earlier in the day.

"Enquiries established that it was actually a result of some filming by the occupants. Those initially arrested were then released."

The spokesman said the police had to take such reports seriously.

There was a bright side for Martin – he was able to get some shots of the police helicopter that he intends to use in the film.