(05-14) 16:46 PDT SAN FRANCISCO -- More than four years after San Francisco defied state marriage laws by allowing nearly 4,000 same-sex couples to wed at City Hall, the state Supreme Court is set to decide today whether gays and lesbians have a constitutional right to marry in California.

But the decision, due at 10 a.m., may not be the last word. Conservative religious organizations have submitted more than 1.1 million signatures for an initiative that would amend the state Constitution to outlaw same-sex marriage. If at least 694,354 signatures are found to be valid, a tally that is due by mid-June, the measure would go on the November ballot and, if approved by voters, would override any court ruling in favor of same-sex marriage.

Californians have already voted once, in 2000, to reaffirm the 1977 state law that defines marriage as the union of a man and a woman. The 2000 initiative, Proposition 22, was not a constitutional amendment.

The marriage case is the most prominent and politically explosive dispute to come before the court in decades. The justices have largely managed to stay out of the public spotlight since 1986, when voters removed Chief Justice Rose Bird and two liberal colleagues who had joined her in overturning nearly all death sentences to come before the court.

The current court, with a 6-1 majority of Republican appointees, has a centrist record on social issues and has ruled in favor of gay-rights advocates in a number of cases, including three decisions in 2005 requiring equal treatment for same-sex parents in disputes over child support and custody. The justices seemed sharply divided at their hearing in the marriage case March 4.

Win or lose, supporters of same-sex marriage have scheduled a "celebration of love and family" at the San Francisco LGBT Center, 1800 Market St., at 5 p.m. today, with similar observances planned in Los Angeles, Sacramento, San Luis Obispo and Palm Springs.

"Express your joy - or frustration - with dignity and resoluteness," the advocacy group Equality California advised participants in an e-mail message.

San Francisco Mayor Gavin Newsom, whose order authorizing the City Hall weddings started the chain of events leading to today's ruling, said the case represents progress, regardless of the result.

"Even if we are not successful, we will have pushed the ball forward," said Newsom, who was testifying to a congressional committee Wednesday in Washington and scrapped a planned trip to Chicago to return to San Francisco today.

The case began in February 2004, when Newsom told the city clerk to issue marriage licenses to same-sex couples, saying he doubted the constitutionality of the marriage law.

The state's high court halted the weddings a month later, then nullified the marriages in August 2004 and ruled that Newsom lacked the authority to override California law.

The court did not rule on the validity of the law, however, and instead referred the issue to lower courts. Constitutional challenges were immediately filed by 23 couples - including some whose marriages had just been annulled - and by the city of San Francisco.

A Superior Court judge in San Francisco declared the ban on same-sex marriage unconstitutional in March 2005. Judge Richard Kramer said the law violated the right to marry the partner of one's choice - a right first established by California's high court in a 1948 ruling allowing interracial marriages - and also constituted sex discrimination.

But a state appeals court upheld the law in October 2006. In a 2-1 ruling, the court said California is entitled to preserve the historic definition of marriage while protecting gays and lesbians from discrimination by granting marital rights to domestic partners, most of whom are same-sex couples.

That is the argument advanced by state Attorney General Jerry Brown's office, which is defending the marriage law in court. Two conservative organizations, the Campaign for California Families and the Prop. 22 Legal Defense and Education Fund, have entered the case and argued that broadening the marriage law to same-sex couples would weaken traditional matrimony.

Massachusetts is the only state whose high court has ruled that the state's Constitution gives same-sex couples the right to marry.

Check SFGate.com today for complete coverage of the state Supreme Court's ruling.