"We believe Colorado voters have decided to take a more sensible approach to how we deal with marijuana in the state," the Denver Post quoted Mason Tvert, a leader of the movement in favour of legalising marijuana, as saying after two Denver television networks reported it had been approved.
Voters in the state of Oregon also faced a similar ballot issue in Tuesday’s election on whether or not to legalise marijuana, but the outcome of the vote there was not immediately clear.
Colorado is one of several states where use of marijuana for medical purposes is already legal.
Supporters of the measure have argued that legalising marijuana for recreational use would deprive criminal drug cartels of profits and would generate substantial tax revenue for state coffers. Opponents argued that legalisation would attract illegal drug dealers from outside the state and lead to increased use of marijuana by young people.
The Colorado measure would allow residents 21 years of age and older to purchase up to one ounce (28 grams) of marijuana at specially-regulated stores. Adults would also be permitted to grow up to six marijuana plants in their homes.
The Washington State measure sets up a system of state-licensed marijuana growers, processors and retail stores, and allows people over the age of 21 to buy up to an ounce for recreational purposes, but they can’t grow it without a licence.
Legalising recreational use of marijuana on a state level sets up a showdown with the US Federal Government, which still considers it illegal.
US Attorney General Eric Holder has said in the past that the Justice Department would “vigorously” prosecute “individuals and organisations that possess, manufacture, or distribute marijuana for recreational use, even if such activities are permitted under state law”.