(Reuters) - Pot smokers formally gained the right to light up in Colorado on Monday as Governor John Hickenlooper signed into effect a controversial ballot measure legalizing marijuana for adult recreational use in what proponents hailed as a "historic day."

Hickenlooper's signature, largely a formality, made Colorado the second U.S. state after Washington to legalize recreational pot use, and put it on a possible collision course with the federal government - which calls marijuana an illegal drug.

"Voters were loud and clear on Election Day," Hickenlooper said in a statement released by his office. "We will begin working immediately with the General Assembly and state agencies to implement Amendment 64."

The ballot measure, approved by a margin of 55 percent to 45 percent, amends Colorado's constitution to legalize the personal use and possession of up to an ounce (28 grams) of pot by adults 21 and over. It also allows users to grow up to six plants at home.

Hickenlooper, a Democrat who had opposed the amendment but said he would respect the will of voters, had been required by law to issue the executive order, or "official declaration of the vote," within 30 days of certification by Colorado's secretary of state on December 6.

His move, more than three weeks before the deadline, put the amendment into immediate effect without the pre-planned hoopla seen in Washington state last week when pot users organized a downtown Seattle public weed fest to begin the moment marijuana became legal there.

Eighteen U.S. states and the District of Columbia have already removed criminal sanctions on the use of pot for medical purposes, but Colorado and Washington were the first to allow it for recreational use.

The moves by the two Western states came in defiance of federal law, and experts have said that the victories by pro-marijuana activists could be short-lived if they are fought by the U.S. Department of Justice.

FEDS 'REVIEWING' LAW

Colorado law will ultimately permit cannabis to be commercially grown and sold by state-licensed producers and distributors, and to be taxed, in a system modeled after those used in many states for alcohol sales.

For now, it remains illegal to buy or sell marijuana in any quantity in Colorado. But the governor ordered creation of a task force to recommend details of a sales-and-taxation plan for the state legislature to pass in the near future.

"This is a truly historic day. From this day forward, adults in Colorado will no longer be punished for the simple use and possession of marijuana," Amendment 64 spokesman Mason Tvert said in a written statement.

"We look forward to working with the governor's office and many other stakeholders on the implementation of Amendment 64. We are certain that this will be a successful endeavor and Colorado will become a model for other states to follow," he said.

John Walsh, U.S. Attorney for Colorado, said in a statement that the U.S. Department of Justice was reviewing the Colorado and Washington measures, and that its "responsibility to enforce the Controlled Substances Act" had not changed.

"Regardless of any changes in state law, including the change that will go into effect on December 10th in Colorado, growing, selling or possessing any amount of marijuana remains illegal under federal law," Walsh said.

"Members of the public are also advised to remember that it remains against federal law to bring any amount of marijuana onto federal property, including all federal buildings, national parks and forests, military installations, and courthouses."