The reclassification of cannabis as a Class B drug has come into effect in England and Wales amid complaints the new laws are "illogical".
Ministers went against their advisors to upgrade the drug because of worries about its impact on mental health.
Magistrates welcomed the reclassification but said planned fines for possessing small amounts undermined the more serious classification.
They said it sent the signal cannabis is not as bad as other Class B drugs.
Plans to introduce a "three strikes" system for cannabis possession start with a warning, then an £80 spot fine for a second offence. Scotland and Northern Ireland have opted out of this penalties arrangement for England and Wales, retaining the former system for class B drugs.
Only when a third offence is committed, will the person be liable to arrest and prosecution.
"It's a dual justice system. If you smoke or take another Class B drug you'll be brought to court, if you take cannabis you'll be given a fine. Where's the justice in that?"
But Liberal Democrat MEP Chris Davies said the changes were disproportionate to the harmfulness of the drug and that they would in any case be ineffective.
He told the BBC: "The reality is cannabis has been illegal for 80 years and it has made no difference whatsoever to its usage... The use of cannabis should be a matter of health policy not criminal law.
"The principle of a free society is that people should be able to do whatever they like as long as they cause no harm to other people."
Brian Paddick, a former Metropolitan Police commander, also said the upgrade would be ineffective as the classification of a drug was not a factor drug users take into consideration.
However Debra Bell, who says her son became aggressive and dishonest after first smoking the drug aged 14, told the BBC the age of users is falling and the damage to families is acute.
'Three times stronger'
She told the BBC: "Thousands across the UK who have written to us have talked about the damage to families... our children are our future."
A £2.2m TV, radio and internet campaign will launch next month to warn young people about the dangers of using the drug.
Home Secretary Jacqui Smith decided to reclassify cannabis despite an Advisory Council on the Misuse of Drugs' review - commissioned by Gordon Brown - saying it should remain Class C.
Ms Smith said stronger "skunk" varieties account for 80% of the cannabis seized on the streets, and that the drug is nearly three times stronger than in 1995.
Liberal Democrat home affairs spokesman Chris Huhne said: "The move to Class B has got nothing to do with public health and education and everything to do with posturing on penalties.
So-called because it has a very strong smell
Contains much higher levels of the active ingredient - tetrahydrocannabinol (THC).
Now accounts for between 70% and 80% of samples seized by police
Six years ago it accounted for 15% of samples
"This farce would have been avoided had ministers heeded the advice of the experts on the Advisory Council on the Misuse of Drugs."
The advisory council's report, Cannabis: Classification and Public Health, described the drug as a "significant public health issue".
But it said it should still remain a Class C drug, saying the risks were not as serious as those of Class B substances such as amphetamines and barbiturates.
Class C includes substances such as tranquilisers, some painkillers, GHB (so-called "liquid ecstasy") and ketamine. Possession of Class C drugs is treated largely as a non-arrestable offence.
The Conservatives have said the government's reversal of its earlier decision showed the downgrading of cannabis had been a mistake.
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