The downgrading of cannabis on Thursday will lead to a rise in the number of drug addicts and criminals, police officers' leaders have warned.
Smoking cannabis in public could still lead to arrest
The Police Federation of England and Wales said many people were confused and believed the drug was now legal.
Cannabis has been downgraded from Class B to C, which means police will no longer automatically arrest those caught in possession of it.
The maximum penalty for possession is cut from five to two years in prison.
The government said the re-classification will bring credibility to its drugs strategy and give police more time to tackle hard drugs.
But the Tories say the policy is "muddled" and that they would overturn it.
Police Federation chairwoman Jan Berry said: "Many users who progress to hard drugs admit they started on cannabis.
"I am deeply worried that many people will see the reclassification of cannabis today as decriminalisation and we will see a rise in the number of users finding themselves drawn into a life of drugs and crime."
The federation said it first raised its concerns about the downgrading in October 2002.
And it said there were better ways to deal with the amount of time spent policing cannabis use.
"I would rather have seen the time and energy given to reclassify cannabis used more effectively for education and treatment programmes," Ms Berry said.
The re-classification gives cannabis the same status as anabolic steroids and anti-depressants.
Police have been given new powers to arrest people for possession of Class C drugs.
But the Association of Chief Police Officers (Acpo) has issued guidelines which say arrests should only be made in aggravated circumstances, such as smoking a joint in public or repeat offending.
Most people caught with the drug will be given an on-the-spot warning and have the drug confiscated.
Under-18s will receive a formal warning at a
Ministers said the move is designed to free up police to
target hard drugs like cocaine and heroin which is believed to be responsible for 99% of the £18.8bn a year cost of drug abuse.
A £1m campaign was launched last week to remind people that cannabis is still illegal.
But Tory leader Michael Howard said downgrading was "sending out a confused and muddled signal".
Campaigners for cannabis legalisation have welcomed the downgrading as a step in the right direction.
They said it could cut the number of arrests for possession from about 80,000 a year to 30,000.
Some users are planning to test the new law by opening cannabis clubs and cafes where they can smoke the drug with others.
A demonstration is due to be held in London, arguing that the change does not go far enough.
Doctors' leaders launched a last-minute campaign to stop the cannabis downgrading.
The British Medical Association said it was "extremely concerned" the move would mislead users into thinking it was safe.
"Chronic cannabis smoking increases the risk of heart disease, lung cancer, bronchitis and emphysema," said BMA deputy chairman Dr Peter Maguire.
Mental health charity Rethink called for clear health warnings to be issued over increased risk of developing schizophrenia and other forms of psychosis from cannabis use.