Voters approved an amendment legalizing recreational marijuana use in Colorado on Tuesday, making this one of two states to end prohibition of the drug but also raising new legal questions and setting up potential court battles over the measure’s implementation.

The historic result, projected by The Denver Post Tuesday night, drew raucous cheers and applause from the amendment’s supporters, who gathered in hundreds at Casselman’s in downtown Denver.

“We won! We won!” supporters cried as the results were splashed across a giant screen.

Amendment 64 led late Tuesday night with 53.3 percent voting yes and 46.7 percent voting no, with 1,863,535 votes or about 66 percent of active voters counted, according to the Colorado Secretary of State’s office.

“This is really groundbreaking,” said Beau Kilmer, co-director of the Rand Drug Policy Research Center. “No modern jurisdiction has ever removed the prohibition on the production and possession of marijuana for recreational purposes. … Since no one has done this before, there are a lot of uncertainties.”

Voters in Washington state approved a similar measure Tuesday; in Oregon a legalization issue failed.

The amendment will allow those 21 and older to purchase up to one ounce of the drug at specially regulated retail stores. Possession would be legal but not public use.

Adults could grow up to six marijuana plants in their homes. It also sets up a direct challenge to federal drug law, which regulates it as an illegal substance. Federal authorities have not said how they will respond.

“This demonstrates that the people of Colorado are just as smart as we thought they were,” said Mason Tvert, one of the directors of the Yes on 64 Campaign. “They were fed up with prohibition and decided they want a more sensible approach.”

Tvert and other supporters of the measure have said it will generate tens of millions of dollars in tax revenue for state and local governments. They said it would shed light on the black-market marijuana industry and ultimately case dangerous cartels to wither.

Critics argued that passing the amendment would make Colorado a destination for drug tourists and prompt a federal crackdown. They cited concerns about increased use of the drug by children and more drugged driving.

Among the opponents was Gov. John Hickenlooper, who said in a statement Tuesday night that “the voters have spoken and we have to respect their will. This will be a complicated process, but we intend to follow through. That said, federal law still says marijuana is an illegal drug so don’t break out the Cheetos or gold fish too quickly.”

The smell of the drug wafted through the supporters’ party at Casselman’s Tuesday night. But state criminal penalties for possessing the drug won’t disappear until the election is certified, which could take up to two months.

Questions about regulation and enforcement still abound. The first recreational stores would be slated to open in January 2014 and would be separate from existing medical marijuana dispensaries. Local governments could ban marijuana sales, and employers could still bar employees from using the drug.

The amendment doesn’t spell out the details of how the commercial marijuana industry will be regulated. It leaves that up to the state Department of Revenue, which would oversee the specialty shops. Proponents envision something similar to the state’s system governing medical-marijuana businesses, which involves security requirements, the monitoring of plants as they are grown and shipped and auditors who perform site checks.

“Colorado has a lot of work to do quickly in terms of setting up the appropriate rules and structures,” said Rosalie Pacula, another co-director for Rand’s Drug Policy Research Center. She noted that because the measure is a constitutional amendment, it will have to go back to voters for repairs if there are problems; the legislature will be unable to intervene.

The federal government’s response is anyone’s guess, she said, but it will likely be resistant. Other experts have said federal agents could arrest individual users where local authorities are not, though that would be a departure from their usual focus on large-scale dealers.

In the year before the first stores open, the federal government may choose to challenge the measure in court by arguing that it is pre-empted.

A spokesman for the Colorado U.S. attorney’s office said only that the office would not change its approach to enforcement, which for medical marijuana has included ordering dispensaries near schools to shut down and raids on retailers who allegedly sell to the black-market.

“We are reviewing the ballot initiative and have no additional comment at this time,” spokesman Jeff Dorschner said.

Allen St. Pierre, executive director of NORML, a national group that lobbies for legalization, said he has heard from friends and family already planning trips to Colorado for the “cannabis tourism” that will certainly become popular.

“That sort of enthusiasm is hard not to recognize right off the bat,” he said.

Sadie Gurman: 303-954-1661, sgurman@denverpost.com or twitter.com/sgurman