At least 37 stores in the state were licensed to sell marijuana, the Denver Post reported. Purchasers could choose an edible dose resembling a hard candy as well as the kind for smoking.
While most stores had lines of would-be buyers outside waiting for the big moment, the lines were relatively short, the Post said. At one Denver store, 3D Cannabis, where advocates of legal marijuana made a ceremonial "first purchase" at exactly 8 a.m., reporters outnumbered customers.
Sean Azzariti, an Iraq war veteran who said smoking pot helps him deal with post-traumatic stress, was the first buyer. Azzariti appeared in ads before voters approved the referendum legalizing marijuana.
Brandon Harris, who said he drove from Ohio for the big day, planned to move to Denver.
Police reported few problems.
"It's been pretty smooth, orderly," said Sonny Jackson, a Denver police spokesman. "People were acting respectable. The lines haven't been too long, maybe 20 to 30 people."
Marijuana has been illegal in most countries since the beginning of the 20th century. While medical marijuana has been legal in many U.S. states for some time, Colorado is the first to allow recreational sales.
Washington state approved legal pot in November 2012, at the same time as Colorado. Washington is currently implementing its system of legal sales.
Rob Kampia, executive director of the Marijuana Policy Project, said the group is lobbying legislatures and pushing for referendum votes in several other states.
"It's only a matter of time before lawmakers and voters in more states adopt similar laws regulating marijuana like alcohol," Kampia said. "The dominoes are falling."
The Colorado amendment allows for the "personal use and regulation of marijuana" for adults age 21 and over. It also permits the commercial cultivation, manufacture and sale of pot, effectively regulating cannabis in a manner similar to alcohol.
A customer with a Colorado ID can buy as much as an ounce of marijuana at a time. Someone with an out-of-state ID is limited to a quarter ounce.