Police officials and politicians have reacted furiously after a Green party member who tweeted a "fact-check" of 10 Ukip election promises received a visit from two officers.
Two Cambridgeshire police officers knocked on Michael Abberton's door after he criticised the Eurosceptic party’s policies in a tongue-in-cheek poster.
David Davis, a former Tory home affairs spokesman, said the incident could have a "chilling effect" on freedom of expression.
"This is the opposite of what the traditions of British democracy and free speech are all about," he said.
Cambridge Chief Constable Simon Parr said Tuesday it was wrong for the officers to visit Abberton and has called for a review to be carried out.
“I believe in this instance police attendance was not required and I have asked for our approach to this sort of incident to be reviewed to ensure we do not get involved unless there is clear evidence that an offence may have been committed," he told Cambridge News.
Sir Graham Bright, Cambridgeshire’s police and crime commissioner, has also raised concerns saying he "identified with the public outrage regarding the handling of this issue."
The Huffington Post UK discovered that the complaint was made by Cllr Peter Reeve, Ukip's local Government spokesman and national nominating officer.
Reeve told HuffPost UK that he had reported Green party member Abberton to the police for electoral fraud, because his post did not contain an official declaration that it was Green Party material - required by the Electoral Commission.
He said Ukip were still considering suing Abberton for his tweet. "We have also informed the branch of the police which deals specifically in electoral fraud."
But the vast majority of Twitter users – including even some UKIP supporters - came out in support of Abberton.
City MP Julian Huppert has argued Abberton had a right to express his opinions and that he “cannot see what law he may have broken”.
Although officers admitted they believed Abberton had not broken the law, they asked him to delete the tweet in any case – but said they could not force him to do so.
But the Green Party has since said it will be writing to the Home Secretary to demand an investigation into why the visit took place.
The developments came as Ukip asked police officers to arrest demonstrators for a hate crime if they call their supporters "fascists," branding critics "state funded rabble rousers".
Three of the party's European election candidates said, in a joint statement, that they had asked Sussex Police to arrest "any protestors who call our supporters 'fascists', hurl other abuse or any physical assault, for 'hate crime' or under the Public Order Act" at the Hove meeting on Tuesday night.