By Daniel Wallis

DENVER (Reuters) - Opponents of Colorado's marijuana legalization are expected to file a federal lawsuit on Thursday that will seek to shut down the state's new industry and which they say will accuse participants of violating racketeering laws.

Voters in Colorado chose to legalize the use of recreational marijuana by adults in a landmark 2012 ballot, even as the drug remains federally outlawed. Washington state has also legalized recreational pot, while Alaska and Oregon are following suit.

But the Safe Streets Alliance, a Washington, D.C.-based group which describes itself as a national non-profit aimed at reducing youth drug use and violent crime, said its lawsuit will allege that state and local officials in Colorado are violating federal law by promoting the commercialization of marijuana.

"Safe Streets is asking the federal courts to order Colorado officials to comply with federal law and stop issuing state licenses to deal illegal drugs," it said in an online statement.

The group called for testimony from individuals who feel that they, their relatives, properties or businesses have been hurt by the marijuana industry, and said federal racketeering laws give plaintiffs injured by a commercial drug conspiracy the right to an injunction, treble damages, and attorney's fees.

It said it was also suing "several prominent participants" in Colorado's pot industry, but did not provide details.

In December, Colorado vowed to defend its recreational marijuana laws after they were challenged in the U.S. Supreme Court by neighboring Nebraska and Oklahoma amid complaints its weed was being smuggled across their borders.

While marijuana is classified as an illegal narcotic under federal law, President Barack Obama's administration has given individual states leeway to frame their own rules about its possession and sale.

Supporters of legalization in Colorado said voters made their preference for licensed, regulated marijuana businesses clear at the ballot box, and warned the new lawsuit would only risk driving the trade back into the hands of gangs.

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"Hundreds of millions of dollars in marijuana sales that were previously taking place in a dangerous underground market are now being conducted safely," Mason Tvert, spokesman for the Marijuana Policy Project, the largest U.S. pro-pot policy organization, said in a statement.

"It's hard to imagine why anyone would prefer marijuana be controlled by criminals instead of by tightly regulated businesses. If drug cartels relied on litigation instead of violence, this is the lawsuit they would file."

(Editing by Eric Walsh)