Washington has made history as the first US state to legalise marijuana possession for adult recreational use.
The occasion was celebrated by dozens of users near Seattle's famed Space Needle tower amid blaring reggae music and a haze of marijuana smoke.
The public gathering defied a key provision of the state's landmark marijuana law, which forbids users from lighting up outside the privacy of their homes.
And it underscored mixed messages that law enforcement officials have conveyed about the new statute.
Seattle's city attorney had issued a stern warning that smoking marijuana in public would not be tolerated and that violators faced citations carrying $100 fines.
But this was contradicted by the Seattle Police Department's own instructions to its officers to limit their enforcement actions to warnings, at least for the time being.
Passed by voters last month as a ballot measure called Initiative 502, the new marijuana law removes criminal sanctions for anyone 21 or older possessing 28.5g or less of marijuana for personal recreational use.
It also legalizes possession of up to 0.45kg of solid cannabis-infused goods - like brownies or cookies - and up to 2.4kg of the drug in liquid form.
However, driving under the influence of cannabis, or imbibing in public places, where the consumption of alcohol already is banned, remains illegal.
"If you're smoking in plain public view, you're subject to a ticket," Seattle City Attorney Pete Holmes said at a news conference yesterday.
"Initiative 502 uses the alcohol model. If drinking in public is disallowed, so is smoking marijuana in public."
The new law ultimately will permit cannabis to be legally sold and taxed at state-licensed stores in a system to be modelled after those in many states for alcohol sales.
The state Liquor Control Board, along with agriculture and public health officials, have until 1 December 2013 to set up such a system.
But for now, it remains a crime to sell, cultivate or even share one's own stash, even though the law allows individuals to purchase a limited amount for personal possession.
Little if any of the law's fine points seemed to matter to the mellow and largely middle-aged gathering of about 100 people near the foot of the Space Needle as the statute took effect at midnight.
Mike Momany, 61, said he was forming the Washington State Cannabis Tourism Association to promote marijuana tourism.
Although he has smoked marijuana for 40 years, Mr Momany said he had slowed his intake "because it makes me eat too much".
Another smoker, wearing sunglasses and calling himself "Professor Gizmo," 50, said: "Victory for hemp. If our forefathers could see us now."
Colorado voters approved their own ballot measure in November similar to Washington's, although it goes further by allowing individuals to grow small amounts for themselves.
The effective date for Colorado's law is 5 January.
Both states are among 18 that already have removed criminal sanctions for medical use of marijuana.