Washington State's new law makes it legal for adults to possess up to one ounce of marijuana, but some speculate the federal government will prosecute those who use marijuana on federal land because federal law prohibits marijuana use. NBC's Kristen Dahlgren reports.

With marijuana possession now legal in Washington state, and soon in Colorado too, residents face a confusing mishmash of federal and state laws when it comes to whether and where they can get high.

That's because the federal government still bans pot growing and possession, regardless of what state laws say.

Last night, just hours before legislation legalizing pot in her state went into effect, U.S. Attorney Jenny A. Durkan of Washington warned residents that "growing, selling or possessing any amount of marijuana remains illegal under federal law."

Her words could be a buzzkill for Washington's pot-lovers, yet at midnight -- the moment Washington's law went into effect -- marijuana smokers lit up beneath Seattle's Space Needle, reveling in the joy of living in a state that allows possession of pot, even if state law still says it is illegal to smoke it in public.

"It's too good to be just for the young," Pat Edmonson, 67, of Whidbey Island, Wash., said as she smoked marijuana in Seattle's City Center with a crowd of about 100 others who were lighting up, despite the no-pot-in-public rule.

State leaders have appealed to the Justice Department for guidance.

Jim Seida / NBC News Pat Edmonson, 67, of Whidbey Island, Wash., was in Seattle with her daughter to celebrate the legalization of the possession of marijuana.

Seattle City Attorney Pete Holmes encouraged celebrants to enjoy their highs inside closed doors.

"I think that they should acknowledge this newfound right," he told NPR station KUOW. "I think they should celebrate in the privacy of their homes if they choose to do so. And be thankful that we’re no longer arresting some 10,000 Washingtonians a year in the state of Washington and spending well over $100 million in law enforcement resources on that."

In Colorado, a measuring legalizing marijuana use and possession for those over 21 will go into effect next month. But one place where federal laws will have an impact: college campuses.

"In order not to lose federal funds, we need to comply with federal law," University of Colorado at Boulder spokeswoman Malinda Hiller-Huey told The Denver Post.

College students on campuses across the state will be issued criminal tickets if they are found with marijuana, The Post reported. Off-campus, however, students of legal age will be able to grow and use small amounts of marijuana, per the new amendment, according to the University of Colorado.

While Colorado's new weed measure doesn't have any provisions about driving under the influence built into it, Washington state will have a zero-tolerance policy.

"We've had decades of studies and experience with alcohol," Washington State Patrol spokesman Dan Coon told The Associated Press. "Marijuana is new, so it's going to take some time to figure out how the courts and prosecutors are going to handle it. But the key is impairment: We will arrest drivers who drive impaired, whether it be drugs or alcohol."

It's unclear whether the Justice Department will try to stop the decriminalization of pot in Washington and Colorado. The laws in both states allow adults 21 and older to possess a small amount of marijuana, which will be sold in state-licensed stores and taxed heavily, potentially bringing in hundreds of millions of dollars a year for school, health care and government needs.

Before the vote passed in his state, Colorado Gov. John Hickenlooper acknowledged the legal challenges his state would face.

"It's probably going to pass, but it's still illegal on a federal basis. If we can't make it legal here because of federal laws, we certainly want to decriminalize it,” he told NBC’s Brian Williams.

Seventeen states and the District of Columbia already have laws allowing for the medical use of marijuana, according to the National Council of Legislatures. The measures in Washington and Colorado go a step further, explicitly allowing people to smoke pot for more than just medicinal purposes.

NBC News' Pete Williams, Isolde Raftery and Jim Seida contributed to this report.

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