Britain's clowns fear their good name has been tarnished after police forces around the country revealed incidents of people in costume scaring members of the public.

The copycat craze – believed to have been started by a man known on Facebook as the Northampton Clown – involves people dressing as clowns to surprise passers-by in public places.

Several police forces have issued warnings after reported sightings and now details released under the Freedom of Information Act have revealed the extent of the trend.

These include reports to Derbyshire police of a clown carrying a knife and incidents in South Yorkshire of a clown staring through the windows of people's homes. Those two forces recorded 29 and 28 clown-related incidents respectively.

Tony Eldridge, secretary of Clowns International which represents the entertainers in Britain, said the situation had escalated beyond harmless fun.

Eldridge, whose clown name is Bluebottle, said: "This is doing clowning no favours and is harming society.

"The people behind it might see it as a bit of a laugh, but for the victims it can be a horrible experience.

"The fear of clowns – coulrophobia – is a real thing and some people will react very badly to this. Not to mention people who are elderly or vulnerable.

"This has nothing to do with clowning, it's a small group of people with stupid views and it spoils the fun for everybody else."

Most legitimate entertainers followed a code of clown conduct which included not wearing their costume in public, he said.

"We have to reclaim clowning as a positive thing which brings happiness."

The trend first emerged when the Northampton Clown rose to prominence over the summer, gaining nearly 200,000 "likes" on Facebook.

The person responsible posts photos of himself in public places. He most recently uploaded a photo taken outside a brewery in the town on 13 December.

He defended his behaviour, writing on Facebook: "Yes, there have been copycats, but that's not me."

The craze has spread with police in Lancashire reassuring the public after speculation about sightings involving a Stephen King-style clown.

A force spokesman said: "We have had no first-hand reports of any incidents of violence or intimidation."

Last month Norfolk Police told members of the public to ignore clowns in the street after a number of sightings around King's Lynn.

These included a person in a "full clown outfit" with a red suit and red hair chasing members of the public.

Superintendent Carl Edwards said no one had been injured or assaulted.

Dressing as a clown was not illegal, he said, but the force would offer those behind the make-up "strong words of advice".

Sally Beadle, also known as Crazy Bananas, works as a clown in the King's Lynn area.

She knows somebody who was chased by a clown in Downham Market, Norfolk, and said they were left shaken by the experience.

"Before this happened I would pop into the petrol station in costume on my way to a job but now I can't do that," she said. "Even my own children who grew up around clowns have been scared by this.

"It's more than just a job, it's something we love, but people's reaction to us has changed – I was getting messages on Facebook asking if I was responsible for scaring people.

"This is my business and I don't like frightening people."

Responding to the freedom of information request, South Wales police confirmed it had received a report of somebody acting suspiciously while wearing a clown mask.

Derbyshire police received 29 reports of clown sightings after social media claims that a man in the Nottingham area was dressing up and carrying balloons and a knife.

One report read: "Caller scared as someone had tried to get in to their house – posted on Facebook that a clown had been going around village trying door handles."

Another said: "Caller reported having received numerous calls from concerned parents about a clown going up to the windows of houses brandishing a gun and knife frightening the residents."

South Yorkshire police recorded 28 incidents involving people dressed as clowns.

These included a clown standing in a park, jumping out at somebody in the street and staring through the window of a house.

Other police forces also recorded incidents involving people dressed as clowns, although not all were linked to the craze.

Sussex police received reports of three incidents involving the characters, including a speeding offence involving a motorcyclist in full clown costume.