BOSTON — A voter-approved law makes it legal to possess marijuana next week but buyers and sellers still face arrest — and possible prosecution.

The law allows residents 21 and older to have up to an ounce of marijuana in public and up to 10 ounces at home, and to grow up to 12 plants on their property. It also authorizes retail sales and growing facilities, but those aren’t expected to open until 2018 or later.

Law enforcement from State Police to local departments say they’ve received little direction from state officials ahead of the date when the law takes effect, on Dec. 15.

“There’s a lot of confusion because it’s going to be legal to possess something that you can’t legally purchase yet,” said Mark Leahy, executive director of the Massachusetts Association of Police Chiefs. “We’ll have to see how this all plays out because there’s been no guidance.”

Under current law, possession of an ounce or less of marijuana is considered a civil infraction that carries a fine up to $100. Buying and selling pot is illegal, regardless of amount.

The Class D controlled substance also remains illegal under federal law.

Leahy said he expects police to continue making arrests for street sales of marijuana, deferring to prosecutors about whether to press charges.

“We don’t have any choice but to enforce the law as it exists and let the courts decide what to do,” he said. “That’s the only approach we can take because we can’t turn our back on it.”

It’s unclear if district attorneys will pursue cases for small pot deals. A spokeswoman for Attorney General Maura Healey said her office is “closely monitoring the situation” but hasn’t issued guidelines to prosectors.

Jim Borghesani, spokesman for the Campaign to Regulate Marijuana Like Alcohol, which pushed legalization, urges police to exercise discretion between when the law takes effect and when state-regulated retail sales are up and running.

“I don’t think it would be wise to arrest or prosecute people for something that is legal,” he said. “It’s not going to go anywhere in courts, so it would be a big mistake to bring cases forward.”

Borghesani said other states that legalized marijuana — including Colorado, Oregon, Alaska and Washington — experienced “gray areas” when possession was legal before sales were, and police did not target street-level deals.

“If they catch someone with 10 pounds, it’s different,” he said. “That’s trafficking and illegal.”

Legalization advocates worry about talk on Beacon Hill of pushing back the timeline for opening retail shops, which would prolong the uncertainty, he said.

“We think the timelines work,” he said. “We put them into the law for the very reason, that we didn’t want this to turn into the rollout of medical marijuana, which took years to get going.”

Question 4 passed with more than 53 percent of the vote, even with bipartisan opposition that included Republican Gov. Charlie Baker and Democratic Attorney General Maura Healey, as well as an organized, anti-legalization campaign.

Lt. Edward Guy, a spokesman for the Andover police, said his department has been sending officers to train on the new law’s requirements and how to detect impaired drivers.

Unlike a Breathalyzer that detects blood alcohol levels, there is no device for gauging marijuana impairment.

“It’s a strange situation because there are so many unknowns,” he said. “It’s a new world.”

As for street-level sales, Andover police will continue to make arrests and refer cases to the Essex County district attorney office, Guy said.

“Marijuana is still illegal under federal law,” he said. “That’s the way we’re going to look at it.”

Matt Allen, field director for the Massachusetts chapter of the American Civil Liberties Union, said voters who approved Question 4 “clearly sent a message that they wanted to see an end to the costly war on drugs … because it doesn’t work.”

“Arrests and prosecutions for marijuana offenses create criminal records that are barriers to housing, education and work,” he said. “Law enforcement should be looking at the big picture and not finding loopholes to continue going after people for low-level marijuana offenses.”

Allen said arrests for pot possession dropped nearly 93 percent in the six years after the state decriminalized marijuana possession under an ounce in 2008.

But as of two years ago, he noted, African-Americans were arrested for pot possession at a rate 3.3 times higher than whites, even though use is similar across racial and ethnic groups.

Peter Elikann, a Boston-based defense attorney, said the state needs to quickly regulate marijuana sales to clear up any confusion.

“Eventually there will be regulations on the sale of marijuana, similar to alcohol and cigarettes,” he said. “But until then it’s going to be hard to prosecute, because the law is so vague.”

Christian M. Wade covers the Massachusetts Statehouse for North of Boston Media Group’s newspapers and websites. Email him at cwade@cnhi.com.