Washington DC Mayor Vincent Gray is pictured at a news conference on Capitol Hill in Washington, Tuesday, Jan. 4, 2011. (AP Photo/Charles Dharapak)

(Adds quote from spokeswoman, details on measure)

WASHINGTON, March 31 (Reuters) - District of Columbia Mayor Vincent Gray on Monday signed a bill that decriminalizes possession of up to an ounce (28 grams) of marijuana in the U.S. capital.

The law makes possession a civil violation with a penalty of $25, lower than most city parking tickets. Possession had been a misdemeanor carrying up to six months in jail and a $1,000 fine.

"He did sign it this morning," said spokeswoman Doxie McCoy. Gray is facing a tight race in the Democratic primary on Tuesday after being tarnished by links to a campaign finance scandal.

Proponents had backed the marijuana measure as an issue of fairness. A study by the American Civil Liberties Union had shown that blacks in Washington were eight times more likely to be arrested for pot than people of other races.

Decriminalizing possession of small amounts of marijuana in the District of Columbia is part of a nationwide trend to lessening penalties for marijuana. Colorado and Washington state legalized recreational marijuana use in voter initiatives in 2012.

Marijuana remains illegal under federal law, and the District's new measure will face review by the U.S. Congress, which is granted the power to oversee local laws by the U.S. Constitution.

The measure will now face a 60-day congressional review. (Reporting by Ian Simpson; Editing by Scott Malone and Steve Orlofsky)