The Washington law legalizes possession of up to 16 ounces (0.45 kg) of solid cannabis-infused goods - like brownies or cookies - and up to 72 ounces (2.4 kg) of weed in liquid form.
But driving under the influence of cannabis or imbibing in public places where the consumption of alcohol is already banned remain illegal.
"If you're smoking in plain public view, you're subject to a ticket," Seattle City Attorney Pete Holmes told a news conference on Wednesday. "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 modeled after those in many states for alcohol sales. The state Liquor Control Board, along with agriculture and public health officials, have until next December to set up such a system.
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.
Ironically, an early court challenge of the law came from a medical marijuana patient in Olympia, who filed suit last week seeking to block enforcement of a new standard for marijuana impairment while driving, similar to the blood-alcohol standard for drunken driving.
The plaintiff, Arthur West, says the new legal limit - 5 nanograms per milliliter of blood of THC, pot's active ingredient - would unfairly subject him to prosecution for a THC level at which he routinely drives without impairment. A hearing on his request for an injunction was set for Friday.
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.