In the city of Seattle pro-cannabis campaigners celebrated in a haze at the foot of the Space Needle tower at one minute past midnight, the moment the state's new law came into effect.

Vivian McPeak, director of Seattle's annual Hempfest, said: "This is a big day because all our lives we've been living under the iron curtain of prohibition. The whole world sees that prohibition just took a body blow."

The new law only allows cannabis to be smoked inside, and doing so in public is still subject to a $100 fine.

However, the Seattle Police Department told its 1,300 officers that until further notice they shall not issue tickets, and no officers were present at the Space Needle event.

On the city's police website, spokesman Jonah Spangenthal-Lee said: "The department's going to give you a generous grace period to help you adjust to this brave, new, and maybe kinda stoned world we live in."

He added: "The police department believes that, under state law, you may responsibly get baked, order some pizzas and enjoy a Lord of the Rings marathon in the privacy of your own home, if you want to."

The department also posted a picture of actor Jeff Bridges as the cannabis-smoking character "The Dude" from the comedy film "The Big Lebowski".

Encouraging indoor cannabis smoking, it carried the caption: "The Dude abides, and says, 'take it inside!"

Washington and Colorado became the first two states decriminalise and regulate the possession of cannabis in ballots held alongside the US presidential election on Nov 6. The Colorado law takes effect on Jan 5.

In Washington it is now legal for adults over the age of 21 to possess an ounce of the drug, or up to 16 ounces of cannabis-infused goods like brownies or cookies, or up to 72 ounces in liquid form.

Growers and processors of cannabis will be regulated and the drug will be sold in licensed shops.

It will be subject to a 25 per cent tax at each stage of that process. The move is expected to bring hundreds of millions of dollars in tax revenue for spending in areas including schools and health care. The establishment of the regulation and tax system will take another year.

However, with cannabis still illegal under United States law, through the Controlled Substances Act,