With 100 percent of precincts reporting early Wednesday, 54 percent, or 56,001 voters, cast ballots for the ordinance, while 46 percent, or 48,632 voters, voted against it.
Under the measure, residents over 21 years old could possess up to an ounce of marijuana in Denver, which is nicknamed the Mile High City for its elevation.
"We educated voters about the facts that marijuana is less harmful to the user and society than alcohol," said Mason Tvert, campaign organizer for SAFER, or Safer Alternatives For Enjoyable Recreation. "To prohibit adults from making the rational, safer choice to use marijuana is bad public policy."
Bruce Mirken of the Washington, D.C.-based Marijuana Policy Project said he hoped the approval will launch a national trend toward legalizing a drug whose enforcement he said causes more problems than it cures.
However, many opponents of the measure said it made no sense to prevent prosecution by Denver authorities while marijuana charges are most often filed under state and federal law.
Seattle, Oakland, California, and a few college towns already have laws making possession the lowest law enforcement priority.
The Denver proposal seemed to draw at least as much attention for supporters' campaign tactics as it did for the question of legalizing the drug.
Tvert argued that legalizing marijuana would reduce consumption of alcohol, which he said leads to higher rates of car accidents, domestic and street violence and crime.
The group criticized Mayor John Hickenlooper for opposing the proposal, noting his ownership of a popular brewpub. It also said recent violent crimes — including the shootings of four people last weekend — as a reason to legalize marijuana to steer people away from alcohol use.
The measure would not affect the medical marijuana law voters approved in 2000. In June, the U.S. Supreme Court ruled that medical marijuana laws in Colorado and nine other states would not protect licensed users from federal prosecution.
Also Tuesday, voters in the ski resort town of Telluride rejected a proposal to make possession of an ounce or less of marijuana by people 18 or older the town's lowest law enforcement priority. The measure was rejected on a vote of 308-332.