Police across England have been called to dozens of incidents in which pranksters dress as "creepy clowns" to deliberately scare people.
The culprits are said to be following a trend that started in the US.
A 30-year-old man was arrested and cautioned in Norwich after someone dressed as a clown jumped out from behind a tree and "terrified" a woman in a public park.
On Sunday Thames Valley Police said it was called to 14 incidents in 24 hours.
In the Norwich case, the woman was walking alone in Eaton Park at about 20:30 BST on Sunday when the man leapt out, screamed at her and ran after her.
Supt Lynne Cross, of Norfolk Police, said such incidents "may seem harmless, but it is quite frightening to those who experience it".
In County Durham on Friday, four children were followed to school by a man in a clown outfit who was armed with what turned out to be a plastic machete.
In a separate clowning caper in County Durham on Friday, police in Peterlee posted a photo on their Facebook page of items including two masks confiscated from two 12-year-olds who officers said had gone to a primary school to scare children.
Elsewhere, a 13-year-old boy is currently on police bail after being arrested on suspicion of common assault in West Bromwich on Saturday.
It is alleged they boy, wearing a clown mask, approached a 14-year-old on New Swan Lane at about 16:45 and grabbed his arm.
Why are we scared of clowns?
Professor Mark Griffiths, a chartered psychologist at Nottingham Trent University, says clowns tend to be scary because of their exaggerated looks and evil representation in films.
"The vast majority of people are not scared of clowns day-to-day but a clown's face has become part of a scare culture.
"There is a stereotype of the nasty, evil, eerie clown. If you look at clowns facially what you tend to find is part of their face or feet are exaggerated, they have huge noses, scary mouths and wildfire hair.
"We also have a cinematic trope. If you look at everything from Heath Ledger in Batman to Stephen King's It, we've got these characters with clown faces that are either killing people or doing really nasty things. Even if you have not come into contact with clowns, you're influenced by what you see in television and films."
Meanwhile, in Kidlington, Oxfordshire, a man dressed as a clown and carrying a baseball bat was reported to have chased a 10-year-old child through a park.
Gloucestershire Police said it had received six reports of "clowns" behaving suspiciously or carrying knives. In one instance a child was followed.
A cyclist in Eastbourne, Sussex, was left "shaken" after someone dressed as a clown jumped out from a bush brandishing what he believed was an offensive weapon.
And in Sudbury, Suffolk, a boy was chased by "several people dressed as clowns".
People who have been approached by the clowns have taken to social media to warn others.
A contributor to the Spotted: Loughborough Facebook page said: "I was walking through Shelthorpe cemetery via the footpath next to the school. I was approached by what can only be described as a clown with an axe. I have never been so terrified in my life."
Thames Valley Police said clowns were "tying up resources which could impact on calls to other incidents".
Ch Supt Andy Boyd said: "While we do not want to be accused of stopping people enjoying themselves, we would also ask those same people to think of the impact of their behaviour on others and themselves.
"Their actions can cause fear and anxiety to other people. This could be perceived to be intimidating and threatening which could lead to public order offences, arrest and a criminal record."
The clown craze began in the US in late summer and has since spread to Canada and Australia.
Schools in Texas and Alabama were shut down, while the White House press secretary had to field questions about the president's stance on the phenomenon.
Inspector Simon Starns, of Sussex Police, said: "We will respond if someone feels threatened and the culprit could end up being arrested and then they won't find it so funny."