Liberal Democrats and justice groups have questioned some of the tough sentences handed down to riot offenders after two men were jailed for four years each for attempting to incite a riot on Facebook.

The party's home affairs spokesman, Tom Brake, said sentencing should not be about "retribution".

"Clearly there are cases where offenders who have committed very serious crimes should expect very serious sentences and that is what I expect to happen. But there have been some cases where people who have committed petty offences have received sentences which, if they had committed the same offence the day before the riots, they would not have received a sentence of that nature.

"This should be about restorative justice, in other words making people acknowledge the offences they have committed and preferably if the victims want it, actually sit down face to face with the victims so they can hear from the victims the impact they have had, but it should not be about retribution", he told Newsnight.

Tessa Munt, the MP for Wells, told the Guardian the plans were "bonkers, bonkers, bonkers" and only served to make headlines instead of "calm, rational policy-making".

Another senior Liberal Democrat, the party's home affairs spokeswoman in the House of Lords, Lady Hamwee, also told the paper there should be "zero tolerance for zero tolerance".

Their comments came as a spokesperson for the Howard League For Penal Reform told the Times there was a "complete lack of proportionality" in some cases.

But Eric Pickles, the Communities Secretary, told BBC Radio 5 Live that "these kind of exemplary sentences are necessary".

"I think we need to understand that people for a while thought that this was a crime without consequence. We cannot have people being frightened in their beds, frightened in their own homes for their public safety," he said on Wednesday.

Conservative MP Margot James also defended the sentences, saying she hoped "judges would err on the side of severity for cases like these".

"I think the young men involved were inciting a riot, trying to organise a sort of mayhem that we saw on the streets eight nights ago in Salford, which would have put lives at risk, and at the very least they would have distracted the police from trying to deal with that crisis, and put a lot of fear into people," she told Newsnight.

On Tuesday Jordan Blackshaw, 21, from Northwich, and Perry Sutcliffe-Keenan, 22, from Warrington, were jailed at Chester crown court. on Tuesday.

The men had posted messages to the social network in an attempt to spread disorder that hit London and other cities in the UK last week.

Neither riots occured but the recorder of Chester, Elgin Edwards, said he hoped the sentences would act as a deterrent.

The Crown Prosecution Service defended the sentences, and said that the men had caused "significant panic and revulsion".

"They both used Facebook to organise and orchestrate serious disorder at a time when such incidents were taking place in other parts of the country. Both defendants, in Northwich and Warrington respectively, sought to gain widespread support in order to replicate similar criminality."

Assistant Chief Constable Phil Thompson added that the men had "struck fear into the hearts of communities".

"The sentences passed down today recognise how technology can be abused to incite criminal activity, and send a strong message to potential troublemakers about the extent to which ordinary people value safety and order in their lives and their communities."