A youth charity has accused police and councils of demonising children by highlighting anti-social behaviour ahead of Halloween and Bonfire Night.
Police said they did not want to be branded 'killjoys'
The British Youth Council says the campaigns amount to discrimination and contradict government policy to promote positive images of youth.
BYC officials say it is also a waste of taxpayers' cash because only a "tiny" minority of young people cause trouble.
Police have insisted that officers are responding to genuine public concern.
Cambridgeshire police, along with several local councils, are giving shopkeepers a poster which depicts a group of children as ghouls and monsters and warns young people they could be arrested if found in possession of flour or eggs.
Meanwhile, Lincolnshire police are warning youngsters that "trick or treating" could lead to imprisonment.
But BYC campaigner Rachael Dumigan said: "If you replaced the term 'young people' in these posters with 'black people' or 'Irish people' or 'old people' it would be blatant illegal discrimination.
"I don't know what the figures are but I suspect the percentage of young people who cause trouble is tiny - about the same as the percentage of adults who cause trouble.
"Police officers, who I guess were also young once, would be better putting money into projects to help disadvantaged children instead of demonising all young people in these posters."
The BYC also cites a recent government strategy document warning of the dangers of promoting negative and unfair images of youngsters.
A Cambridgeshire police spokeswoman defended the campaign, saying: "We do not want to be seen as killjoys but we have had genuine concerns expressed by some members of the public about anti-social behaviour on Halloween.
"Last year we received over 60 '999' calls in relation to incidents of eggs being thrown at people and property."
A spokesman for Lincolnshire police added: "Many people, particularly the elderly, dread this time of year and feel anxious and frightened as it approaches."