The West Coast state's liberal marijuana law, passed by voters last month, takes effect on Thursday, removing criminal sanctions for anyone 21 or older possessing 1 ounce (28.5 grams) or less of pot. It also legalizes possession of up to 16 ounces (0.45 kg) of solid marijuana-infused goods - like brownies - and up to 72 ounces (2.4 kg) of weed in liquid form.

Prosecutors in several counties announced last month they were dismissing scores of misdemeanor marijuana possession cases following voter passage of Washington's landmark statute, known as Initiative 502.

But lighting up a joint in public places, where the consumption of alcohol already is banned, will remain illegal.

"If you're smoking in plain public view, you're subject to a ticket," Seattle City Attorney Pete Holmes said during a news conference on the implications of the new law, at the American Civil Liberties Union's Washington state office.

"Initiative 502 uses the alcohol model. If drinking in public is disallowed, so is smoking marijuana in public," Holmes said.

The warning seemed aimed, at least in part, at outdoor celebrations by marijuana advocates planned for midnight on Wednesday and on Thursday evening near Seattle's famed Space Needle tower. Organizers of Thursday's event said they expected a crowd of at least 500 to attend.

Police could ticket public marijuana smokers for infractions that carry fines of about $100, similar to penalties for a traffic citation, Holmes said.

A separate group of about 40 pot enthusiasts planned to gather at midnight on Wednesday, as the law takes effect, outside the office of Seattle Hempfest, an annual three-day "protestival" celebrating marijuana and hemp products that attracted 80,000 attendees in August.

"This is a very big step in a culture that's been pushed under the rug for a very long time," staffer Darby Hageman told Reuters. "It's now the lowest law-enforcement priority, and we plan to celebrate."

STILL A FEDERAL CRIME

Public celebrations also would directly defy federal law, which still classifies marijuana as an illegal narcotic.

In a statement issued on Wednesday, U.S. Attorney Jenny Durkan in Seattle reiterated the U.S. Justice Department position that growing, selling or possessing any amount of marijuana remained a federal crime, regardless of any changes in state law.

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 January 5. Both states are among 18 that already have removed criminal sanctions for medical use of marijuana.

In addition to making it lawful to possess small amounts of pot for recreational purposes, the new laws in Washington and Colorado will permit cannabis to be legally sold and taxed at state-licensed stores in a system modeled after those many states have in place for alcohol sales.

In Washington, the state Liquor Control Board, along with agriculture and public health officials, have until December 1, 2013, to set up such a system.

In the meantime, individuals are permitted to purchase small amounts legalized under the law for personal possession, although cultivation or selling is still outlawed, as is sharing one's personal stash with another individual.

Initiative 502 also set a new standard for marijuana impairment while driving, similar to the blood-alcohol standard for drunken driving, and those provisions will be enforced starting on Thursday.

(Editing by Steve Gorman and Peter Cooney)

• A Permanent Economic Emergency