Signing a petition to home secretary Theresa May, set up by Defend the Right to Protest, Mark Serwotka said:

“Whatever anyone feels about the action itself, the sentence of six weeks in prison for throwing a foam pie is absurdly harsh.

“This is further evidence of an attempt to deter protest and direct action through increasingly harsh sentencing.”

The petition, at www.ipetitions.com/petition/freejonniemarbles, states:

“No one was harmed or put in any physical danger. Murdoch had not wanted to press charges, but the court proceeded anyway.

“District Judge Daphne Wickham, who sentenced Jonnie, is the same person who let Sgt Delroy Smellie off the hook for assaulting Nicola Fisher at the G20 protest where Ian Tomlinson was killed. Wickham ruled that Smellie’s behaviour was lawful despite clear video evidence of him striking Fisher with a baton.

“Jonnie’s excessive sentence for a prank that hurt no one, committed by someone with no criminal record, against a man whose media empire has ruined lives, is yet more evidence of government crackdown on dissent. There are many people who should arguably be in jail for their role in the News International phone hacking affair, but Jonnie is not one of them.

“We call for Jonnie to be released from prison immediately.”