It becomes the sixth US state, and by far the most populous, venturing beyond legalized medical marijuana to permit the sale of cannabis products of all types to customers at least 21 years old.
Colorado, Washington, Oregon, Alaska and Nevada were the first to introduce recreational pot sales on a state-regulated, licensed and taxed basis. Massachusetts and Maine are on track to follow suit later this year.
With California and its 39.5 million residents officially joining the pack, more than one-in-five Americans now live in states where recreational marijuana is legal for purchase, even though cannabis remains classified as an illegal narcotic under US law.
The marijuana market in California alone, which boasts the world's sixth-largest economy, is valued by most experts at several billion dollars annually and is expected to generate at least a $1 billion a year in tax revenue.
"Adding California to the regulated [recreational] market for cannabis is a really big deal," said Heather Azzi, a senior attorney for the Marijuana Policy Project, an advocacy group working to liberalize marijuana laws.
Uruguay became the first and only country to legalize recreational marijuana sales nationally, permitted through its pharmacies starting in July 2017, but is far smaller in comparison, with a population of just 3.4 million.
Still, most California jurisdictions are sitting out the highly anticipated New Year's Day inauguration of recreational cannabis sales.
Many, including Los Angeles and San Francisco, will not be ready for days or weeks because of additional red tape required by city and county governments before would-be retailers can obtain their state licenses.
But business will almost certainly be brisk at newly permitted shops ready on Day One. They number about four-dozen outlets across California, according to an authoritative guide to the cannabis market, GreenState, published by the San Francisco Chronicle.
Stores authorized to carry recreational weed were set to go on New Year's Day in San Diego, San Jose, Santa Cruz, Oakland, Berkeley, Eureka and Desert Hot Springs, among other locales. Hundreds more are expected to open throughout the state as the year progresses.
Among the very first will be the Oakland-based Harborside dispensary, which has long ranked as the largest US medical marijuana outlet. It planned to opens its doors at 6 a.m. local time on Monday.
Customers in the recreational sector - which state regulators prefer to call the "adult use" market - are only permitted to buy an ounce (28 grams) of raw cannabis or its equivalent at a time.
Medical patients can buy unlimited quantities, but must present a doctor's note and have purchased a medical ID card.
The stage for Monday's grand opening was set when voters passed a ballot measure in November 2016, Proposition 64, immediately legalizing personal possession and use of recreational pot by adults 21 and over.
But it has taken California lawmakers and bureaucrats over a year to devise a licensing, regulatory and tax structure for all phases of the commercial distribution chain.
California in 1996 became the first state to legalize marijuana for medical use, and more than 30 states have since done likewise.
(Reporting by Steve Gorman; Editing by Kim Coghill)
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