BOSTON  Last month, voters approved a statewide measure decriminalizing the possession of small amounts of marijuana. Now, wary authorities say, comes the hard part. They are scrambling to set up a new system of civil penalties before Jan. 2, when the change becomes law. From then on, anyone caught with an ounce or less of marijuana will owe a $100 civil fine instead of ending up with an arrest record and possibly facing jail time.

It sounds simple, but David Capeless, president of the Massachusetts District Attorneys Association, said the new policy presented a thicket of questions and complications.

One of the most basic, Mr. Capeless said, is who will collect the fines and enforce other provisions of the law. For example, violators under 18 will be required to attend a drug awareness class within a year, but it is unclear who will make sure that they do so. The fine increases to $1,000 for those who skip the class.

A complicating factor, said Mr. Capeless, the district attorney in Berkshire County, is that state law bans the police from demanding identification for civil infractions.