SACRAMENTO -- Gov. Arnold Schwarzenegger announced Wednesday he will veto a bill that would have made California the first state in the nation to legalize same-sex marriage through legislative action.

Schwarzenegger said the legislation, given final approval Tuesday by lawmakers, would conflict with the intent of voters when they approved Proposition 22. That measure was put on the ballot in 2000 to prevent California from recognizing same-sex marriages performed elsewhere.

"We cannot have a system where the people vote and the Legislature derails that vote," the governor's press secretary, Margita Thompson, said in a statement. "Out of respect for the will of the people, the governor will veto (the bill)."

Proposition 22 stated that "only marriage between a man and a woman is valid or recognized in California." The bill to be vetoed by Schwarzenegger would have defined marriage as a civil contract between "two persons."

Massachusetts' recognition of gay marriages came through a court ruling.

Gay rights advocates said Schwarzenegger had betrayed the bipartisan ideals that helped get him elected in the 2003 recall.

"Clearly he's pandering to an extreme right wing, which was not how he got elected," said Geoff Kors, executive director of Equality California, one of the bill's sponsors. "He got elected with record numbers of lesbian and gay voters who had not previously voted for a Republican, and he sold us out."

Kate Kendell, executive director of the National Center for Lesbian Rights, said she was not surprised by word of Schwarzenegger's pending veto.

"Any girlie man could have vetoed this legislation," she said, referring to a term Schwarzenegger used previously to mock Democratic legislators. "A real man demonstrating real leadership as governor of the most populous state in the nation would have chosen a different course of action."

The governor has until Oct. 9 to issue the veto. Despite his promise to do so, Schwarzenegger "believes gay couples are entitled to full protection under the law and should not be discriminated against based upon their relationship."

"He is proud that California provides the most rigorous protections in the nation for domestic partners," the statement said.

The Republican governor had indicated in previous statements that he would veto the bill, saying the debate over same-sex marriage should be decided by voters or the courts.

A state appellate court is considering appeals of a lower court ruling earlier this year that overturned Proposition 22 and a 1978 law that first formally defined marriage as a union between a man and a woman. Meanwhile, opponents of same-sex marriages are planning measures on the ballot next year that would place a ban on gay marriages in the state Constitution.

The announcement dampened a celebratory mood among the bill's supporters, who only the night before cheered, hugged and kissed as the state Assembly narrowly sent the bill to the governor's desk.

San Francisco Mayor Gavin Newsom said Schwarzenegger had missed "a golden opportunity to stand on history and do something that was noble and appropriate." Newsom, a Democrat, had thrust himself into the center of the debate in 2004 by sanctioning same-sex marriages in the city. The state Supreme Court later voided the unions.

"It disappoints me greatly, and it will disappoint literally hundreds and hundreds of thousands of San Franciscans, not to mention millions of people across the country," Newsom said in response to Schwarzenegger's expected veto.

The bill passed the Legislature through the persistence of its author, Assemblyman Mark Leno, a San Francisco Democrat who is one of six openly gay members of the California Legislature.

His original bill had failed in the Assembly by four votes in June, but he then amended it to another bill in the Senate, which voted to approve it last week. It was that amended bill the Assembly passed by a bare majority on Tuesday. Four Democrats who did not vote on the bill in June provided the winning margin this week.

Leno said he requested a meeting with the governor Wednesday to argue his case for the bill but said Schwarzenegger's office did not respond.

"It would certainly be the respectful thing to do. The Legislature has done the hard work of picking up the issue of the day, holding public hearings, having public debate and making a decision," Leno said. "(A veto) would be an enormous disregard for the deliberation of both houses and the millions of people who wish him to sign the bill."

The vote that sent the bill to the governor made the California Legislature the first legislative body in the country to approve of same-sex marriage. As in Massachusetts, civil unions in Vermont were granted through court rulings.

"I'm encouraged that the governor is going to stop the runaway Legislature, and he's going to represent the people," said Karen England of the Capitol Resource Institute, a Sacramento group that lobbied against the bill. "I think Assembly member Leno wanted to rally everyone on his side and he's done exactly the opposite. He's forced his agenda on the rest of us. But in California the votes of the people do matter."