David Cameron has encouraged judges to continue giving tough sentences for people convicted of rioting offences, even as a backlash against the 'zero tolerance' approach taken by some magistrates continues to grow.

Responding to critics who reacted angrily on Tuesday after two men were jailed for four years for unsuccessfully attempting to incite riots on Facebook, the prime minister said that criminal acts "would not be tolerated" by the public.

Cameron said: "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."

"They decided in that court to send a tough sentence, send a tough message, and I think it's very good that courts are able to do that."

Critics of the courts were given fresh ammunition Wednesday, however, after a 17-year-old college student was spared jail for attempting to incite friends to riot on Facebook by Bury St Edmunds youth court.

The boy wrote a message on the site which said: "It's about time we stood up for ourselves for once. So come on rioters – get some. LOL." Some of the boy's 400 friends responded to the message, some just to call him an idiot.

The student was banned from social networking sites for 12 months and will serve 120 hours community service and a 12-month rehabilitation order. He will also live under a 7pm curfew for 12 weeks.

But compared to the four year sentences imposed on the two men in Chester the sentence will be seen by some as inconsistent.

The boy's solicitor Paul Booty said that there had been a "knee-jerk reaction" to the riots, and asked for the magistrates to rise above the pressure to impose custodial sentences on every defendant.

He said: "My concern, not necessarily for this case but for cases up and down the country, is that there will also be an awful lot of appeals."

The boy told magistrates that the messages on Facebook were not intended to be serious. He said: "I meant it as a joke which is why I wrote LOL at the end."

Liberal Democrats and justice groups have questioned some of the tough sentences handed down to riot offenders. Speaking on Newsnight on Tuesday, 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," he told the programme.

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.