LONDON (AFP) - British Prime Minister David Cameron backed tough sentences for rioters on Wednesday after campaigners raised concerns about two men jailed for four years each for trying to organise unrest on Facebook. The sentences given to Jordan Blackshaw, 20, from Cheshire, and Perry Sutcliffe-Keenan, 22, from Warrington, both in northwest England, were the toughest so far following four frenzied nights of violence in England last week. Prosecutors said the men's online incitement to riot last Tuesday caused "significant panic and revulsion", even though no one responded, because they were on the same night as violence erupted in the nearby city of Manchester. But justice campaigners and defence lawyers said the punishment was not in proportion to the crime, and urged the courts not to overreact as they struggle to process almost 1,200 people charged over Britain's worst unrest for decades. "The rush to send a message out is leading to some very bad sentences, which will be overturned on appeal," said Andrew Neilson, director of campaigns for the Howard League for Penal Reform. He said it was "understandable" that the riots be treated as an "aggravating factor", but added: "In the Facebook case we're talking about four years' jail which would normally be associated with serious and violent offences." The sentences compare with typical punishments of four years for robbing someone with a weapon. During a visit to Cheshire, northwest England, Cameron backed the sentence, saying: "What happened on our streets was absolutely appalling behaviour and to send a very clear message that it's wrong and won't be tolerated is what the criminal justice system should be doing." Meanwhile, Britain's police watchdog on Wednesday cleared the country's former top officer of misconduct over an investigation by his force into phone hacking at the News of the World tabloid.