Colorado and Washington became the first US states to legalise the possession and sale of marijuana for recreational use in defiance of federal law, setting the stage for a possible showdown with the Obama administration.
But another ballot measure to remove criminal penalties for personal possession and cultivation of recreational cannabis was defeated in Oregon, where significantly less money and campaign organisation was devoted to the cause.
Supporters of a Colorado constitutional amendment legalising marijuana were the first to declare victory, and opponents conceded defeat, after returns showed the measure garnering nearly 53pc of the vote versus 47pc against.
"Colorado will no longer have laws that steer people toward using alcohol, and adults will be free to use marijuana instead if that is what they prefer. And we will be better off as a society because of it," said Mason Tvert, co-director of the Colorado pro-legalisation campaign.
The Drug Policy Alliance, a national advocacy group that backed the initiatives, said the outcome in Washington and Colorado reflected growing national support for liberalised pot laws, citing a Gallup poll last year that found 50pc of Americans favoured making it legal, versus 46 opposed.
Early returns showed pro-legalisation votes led with 55pc versus to 44pc opposed with about 60pc of ballots tallied in the state's all-mail-in election system.
The outcomes in Colorado and Washington, which already have laws on the books legalizing marijuana for medical purposes, put both states in further conflict with the federal government, which classifies cannabis as an illegal narcotic.
The US Department of Justice reacted to the measure's passage in Colorado by saying its enforcement policies remain unchanged, adding: "We are reviewing the ballot initiative and have no additional comment at this time."
Separately, medical marijuana measures were on the ballot in three other states, including Massachusetts, where CNN reported that voters approved an initiative to allow cannabis for medicinal reasons.
Supporters there issued a statement declaring victory for what they described as "the safest medical marijuana law in the country." Seventeen other states, plus the District of Columbia, already have medical marijuana laws on their books.
A measure that would have made Arkansas the first state in the South to legalise marijuana for medical purposes appeared headed for defeat by 51pc to 49pc with about 80pc of the vote tallied.
Under the recreational marijuana measures in Colorado and Washington, personal possession of up to an ounce (28.5 grams) of marijuana would be legal for anyone at least 21 years of age. They also will permit cannabis to be legally sold and taxed at state-licensed stores in a system modelled after a regime many states have in place for alcohol sales.
Oregon's initiative would have legalized state-licensed sales, as well as possession and cultivation of unlimited amounts of pot for personal recreational use.
The Colorado measure will limit cultivation to six marijuana plants per person, but "grow-your-own" pot would be still be banned altogether in Washington state.
Tvert said provisions legalizing simple possession in Colorado would take effect after 30 days, once the election results are certified. Colorado's amendment also mandates establishing rules for sales and excise tax collections once the state legislature reconvenes in January.
"The voters have spoken and we have to respect their will," Colorado Governor John Hickenlooper, a Democrat who opposed the measure, said in a statement. "This will be a complicated process, but we intend to follow through."
He added: "Federal law still says marijuana is an illegal drug, so don't break out the Cheetos or gold fish too quickly."
The Obama administration has recently pressed an enforcement crackdown against pot dispensaries and greenhouses deemed to be engaged in large-scale drug trade under the pretense of supplying medical cannabis patients in California and elsewhere.
Before Tuesday's election, the administration had been largely silent on latest state ballot initiatives seeking to legalize recreational pot for adults.
Several former US Drug Enforcement Administration directors had urged Obama officials to come out forcefully against the measures, as US Attorney General Eric Holder did when he criticized a 2010 California pot legalisation referendum that was soundly defeated by voters.
Critics say the social harms of legalising pot - from anticipated declines in economic productivity to a rise in traffic and workplace accidents - would trump any benefits.
Backers point to potential tax revenues to be gained and say anti-pot enforcement has accomplished little but to penalise otherwise law-abiding citizens, especially minorities.
They also argue that ending pot possession prosecutions would free up strained law enforcement resources and strike a blow against drug cartels, much as repealing alcohol prohibition in the 1930s crushed bootlegging by organised crime.
"It's no worse than alcohol, and it's widely used in Colorado anyway," said Jean Henderson, 73, a retired resident of Broomfield, explaining her vote in favor of legalisation. "The state can benefit from the taxes rather than put people in jail."