By Li Hongmei

Customers began lining up before dawn on Wednesday to take part in what they called a historic departure from drug laws focused on punishment and prohibition. Many had flown in or driven hours specifically to buy a bag of marijuana.

At the Medicine Man dispensary in Denver, which claims it is the closest marijuana retailer to the airport, as many as half the customers were from out of state, here for the first day that marijuana could be sold legally in Colorado.

The store's owner, Andy Williams, said he had redesignated about 60 percent of his medical marijuana to be sold retail but worried that it would not be enough to meet the demands of the lines snaking out the door.

Despite the long lines, Mr. Williams said, people seemed thrilled to be able to walk into a shop, lay down $50 or $60 and openly buy the drug.

"This is Independence Day for the marijuana community," he said. "People don't like breaking the law. The burden has been taken off them. "

With security guards posted outside many stores and police and state officials watching closely, the day's first sales appeared to go smoothly, officials said. "So far so good," said Ron Kammerzell, director of enforcement for Colorado's Department of Revenue.

Mr. Kammerzell said the state had eight investigators checking retailers' licenses, inspecting packaging and labeling, and ensuring that stores checked each customer's identification to see if they were 21 or older.

To supporters, Wednesday was a watershed moment in the country's tangled relationship with the ubiquitous recreational drug. They celebrated with speeches, hailing it as akin to the end of Prohibition, albeit with joints being passed instead of champagne being uncorked.

To skeptics, it marked a grand folly, one they said would lead to higher drug use among teenagers and more impaired drivers on the roads, and would tarnish the image of a state whose official song is John Denver's "Rocky Mountain High." The governor of Colorado and the mayor of Denver both opposed legalization, and stayed away from the smoky celebrations on Wednesday.

While some 20 states allow medical marijuana, voters in Colorado and Washington State decided last year to go one step further, becoming the first in the nation to legalize small amounts of the drug for recreational use and regulate it like alcohol. Ever since, the states have been racing to devise rules detailing how to grow it, sell it, tax it and track it.

In both Colorado and Washington, recreational marijuana has been legal for more than a year. Adults can smoke it in their living rooms, and eat marijuana-laced cookies without fear of arrest. In Colorado, they are even allowed to grow up to six plants at home. But until Wednesday, dispensaries could sell only to customers with a doctor's recommendation and state-issued medical-marijuana card.

Now, any Colorado resident who is 21 can buy up to an ounce of marijuana at one of the 40 dispensaries that began selling to retail customers on Wednesday. Out-of-state visitors can buy a quarter-ounce, but they have to use it within the state. Carrying marijuana across state lines remains illegal, and the plant is not allowed at the Denver International Airport.