By Ian Simpson

WASHINGTON (Reuters) - Legalization of marijuana in the District of Columbia will go ahead early on Thursday despite a warning from congressional officials that the new standards are unlawful, Mayor Muriel Bowser said.

She noted that voters in the U.S. capital last year overwhelmingly approved Initiative 71, which lets the heavily Democratic city join Washington state, Alaska and Colorado in making marijuana legal for recreational use.

"Our government is prepared to implement and enforce Initiative 71 in the District of Columbia," Bowser, a Democrat, told a news conference flanked by council members, Police Chief Cathy Lanier and city Attorney General Karl Racine.

Her comments came in response to a warning on Tuesday from top Republicans on the House of Representative Oversight Committee that legalization was unlawful and opposed the will of Congress, which has oversight over the District of Columbia.

In a letter to Bowser, committee Chairman Jason Chaffetz of Utah and Mark Meadows of North Carolina, head of a committee subpanel, said a December spending bill had barred the District of Columbia from spending any funds to make pot legal or lessen penalties.

But Bowser and other city officials contend that Initiative 71 was officially certified before the spending ban. The mayor on Tuesday outlined steps that the city was taking ahead of legalization, which takes effect at 12:01 a.m. on Thursday.

Initiative 71 allows adults to possess up to 2 ounces (56 grams) of marijuana and to grow six plants, three of them mature. Sales are barred but transfers of up to 1 ounce (26 grams) is legal.

Bongs, pipes and other paraphernalia are legal but public smoking is not. Marijuana possession is illegal under federal law, and pot is barred from about 20 percent of the district that is federal land.

Congressional opposition has also kept District of Columbia lawmakers from writing rules on how marijuana could be sold, regulated and taxed.

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Bowser said Congress should not worry about District marijuana laws while the Department of Homeland Security faced partial shutdown because of a congressional deadlock over funding.

"Bullying the District of Columbia is not what his (Chaffetz's) constituents expect, nor is it what ours expect," she said.

Asked about Bowser's pledge to move ahead with legalization, Melissa Subottin, a Chaffetz spokeswoman, said: "Consequences do come with violating federal law."

The District of Columbia already has one of the lightest U.S. penalties for pot possession.

(Reporting by Ian Simpson; Editing by Bill Trott and Peter Cooney)