Police are investigating a series of hand-drawn posters put up around West Watford depicting the Prophet Muhammad.
Hertfordshire Constabulary is appealing for information about the posters, which had for "draw Muhammad day" written on them.
The posters, which also carried slogans such as "death to those who insult free speech", appear to be linked to a campaign that started in 2010 relating to the censorship of an episode of the cartoon South Park.
Police Inspector Nick Lillitou said: "We have been made aware of these posters and are investigating their origin. If anyone has information about who put these up they are asked to contact police via the non-emergency number, 101. I’d also urge anyone who has concerns to contact PC Pat Davey, who is Watford’s dedicated Hate Crime Officer on 01923 472085."
The police appeal comes after the posters became an issue in Monday night’s mayoral hustings debate in Watford.
Elected mayor, Dorothy Thornhill, said she felt it was "no coincidence" the posters had appeared at the same time that UK Independence Party was standing in the mayoral elections for the first time.
Yesterday the UKIP candidate, Phil Cox, accused the mayor of using the posters for "political ends".
He added he had been around West Watford yesterday taking down the posters himself before handing them into the police.