Barry Thew, 39, was arrested for wearing a shirt bearing handwritten messages saying “One less pig perfect justice” and “killacopforfun.com” in Radcliffe, near Manchester, northern England, on September 18.
His arrest came hours after two police officers in Manchester were killed in a gun and grenade attack that shocked Britain.
Thew was sentenced to four months in jail on Thursday after earlier pleading guilty to a public order offence. He received another four months for breaching the terms of an earlier suspended sentence.
Civil liberties campaigners called the sentence absurd and warned it would have a chilling effect on free speech.
“It’s time we reform the law to protect freedom of speech and focus the police on bringing to justice those who seek to incite harm, not those who cause offence,” said Nick Pickles, director of civil liberties campaign group Big Brother Watch.
But Inspector Bryn Williams, of the Radcliffe neighbourhood policing team, called Thew’s T-shirt appalling and welcomed the sentence.