BOSTON -- A new state law that says it is no longer a criminal offense in Massachusetts to possess a small amount of marijuana went into effect on Friday.

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A referendum, approved by voters in November, makes possession of less than an ounce of marijuana a civil offense with a $100 fine.

Existing laws that prohibit distributing marijuana or operating a motor vehicle under its influence remain unchanged.

Supporters of the law say it will spare thousands from having a criminal record, which can make it harder to get a job, student loan or gain access to public housing. They also said it will save taxpayers $30 million in costs associated with marijuana arrests.

But critics said marijuana is a gateway drug and the law could lead to more drug use among young people.

Under the new law, anyone under 18 caught with an ounce or less of marijuana must complete a drug awareness course or face a $1,000 fine. Parents or legal guardians will also be notified.

As the law takes effect, state officials are urging cities and towns to ban public pot smoking, similar to public drinking ordinances.

Massachusetts is the 12th state in the country to decriminalize the possession of small amounts of marijuana.