"Honestly, I thought I'd never see the day," said Errin Reaume of Denver, who shared hits of concentrated marijuana at a 1920s-themed "Prohibition Is Over" party in downtown Denver.
Across town, a dispensary was setting up a food truck and coffee service for campers awaiting the opening of sales on so-called Green Wednesday.
Activists hope the marijuana experiment will prove that legalisation is a better alternative than the costly US-led war on drugs. Sceptics worry the industry will make the drug more widely available to teenagers, even though legal sales are limited to adults over 21.
Preparation for the retail market started more than a year ago, soon after Colorado voters in 2012 approved the legal pot industry. Washington state has its own version, which is scheduled to open in mid-2014.
Advocates, who had long pushed legalisation as an alternative to the lengthy and costly global drug war, had argued it would generate revenue for state coffers and save money by not having to lock up so many drug offenders.
Colorado set up an elaborate plant-tracking system to try to keep the drug away from the black market, and regulators set up packaging, labelling and testing requirements, along with potency limits for edible pot.
The US Justice Department outlined an eight-point slate of priorities for pot regulation, requiring states to keep the drug away from minors, criminal cartels, federal property and other states to avoid a federal crackdown.
Police in the eight Colorado towns allowing recreational pot sales were stepping up patrols to dispensaries in case of unruly crowds. Denver International Airport placed signs on doors warning fliers they could not take the drug home in their suitcases.
With the additional police patrols, the airport warnings and various other measures, officials are hoping they have enough safeguards in place to avoid public health and safety harm.