SAN FRANCISCO -- The San Francisco City Attorney's office will launch a legal challenge to the validity of a ballot measure banning gay marriage if passes, a spokesman said Wednesday.

Spokesman Matt Dorsey said City Attorney Dennis Herrera will file the legal challenge in the California Supreme Court if the measure passes.

With 95 percent of precincts reporting, Proposition 8 is leading with 52 percent of the vote. But there are still as many as 3 million ballots left to be counted.

The pending ban dampened the mood of a raucous Barack Obama victory celebration in the Castro, San Francisco's iconic gay neighborhood.

Thousands of people descended Tuesday night on a neighborhood festooned with signs urging a no vote on Proposition 8, which would ban same-sex marriages in California.

The crowd swarmed from the neighborhood's many bars into the streets immediately after Obama was declared the presidential winner at 8 p.m. PDT, and police cordoned off a block in the heart of the district as disco music coursed through giant speakers and gay leaders such as Mark Leno, a Democrat newly elected to the state Senate, climbed a stage on a closed street to whip up enthusiasm for Proposition 8's defeat.

But by midnight, the celebration was muted. As revelers dispersed and the locals returned to the bars early Wednesday, Proposition 8 was leading.

"It's very disappointing," Michael Walker said outside the Moby Dick bar, resigned that the measure was going to win. "It's discrimination."

Others vowed to continue fighting for the right to marry if Proposition 8 did pass. "My view of America is different today," said Diallo Grant, a gay man with mixed-race parents. "The culture wars will continue."

The night had started out much more optimistically for many who believed that a large Democrat-voter turnout would help defeat Proposition 8. Many said they viewed an Obama victory as more important as the vote on gay marriage. "You have to believe things are going in the right direction," said Jacob Logsdon, 25. "That's for the greater good."

It has been five months since the state Supreme Court legalized gay marriage and some in the Castro could not fully enjoy the Obama victory with Proposition 8 still hanging in the balance. "It's bittersweet," said Mark Stephan.

The race was the focus of interest not only in California, but across the nation. If Proposition 8 passes, it would be the first time voters anywhere had closed the door on gay marriage after it had become legal.

Spending for and against the amendment has reached $74 million, making it the most expensive social-issues campaign in U.S. history and the most expensive campaign this year outside the race for the White House.

Christian conservatives who sponsored the measure and gay rights groups working to defeat it have said whichever side wins would score a majority victory in the nation's culture wars, gaining momentum to influence the pace at which other states sanction same-sex unions.

Although more than 61 percent of California voters approved of the marriage law the Supreme Court threw out, leaders of the No on 8 campaign were hopeful public attitudes about sexual orientation and civil rights had changed substantially enough in eight years.