Two dozen gay couples then sued, along with the city and gay rights organizations.

Thursday's ruling could alter the dynamics of the presidential race and state and congressional contests in California and beyond by causing a backlash among conservatives and drawing them to the polls in large numbers.

A spokesman for Republican John McCain, who opposes gay marriage, said the Arizona senator "doesn't believe judges should be making these decisions." The campaigns of Barack Obama and Hillary Rodham Clinton said they believe that the issue of marriage should be left to the states.

Ten states now offer some form of legal recognition to same-sex couples — in most cases, domestic partnerships or civil unions. In the past few years, the courts in New York, New Jersey and Washington state have refused to allow gay marriage.

Outside the San Francisco courthouse, gay marriage supporters cried and cheered as news spread of the decision. Jeanie Rizzo, one of the plaintiffs, called Pali Cooper, her partner of 19 years, via cell phone and asked, "Pali, will you marry me?"

Shannon Minter of the National Center for Lesbian Rights said same-sex marriage advocates could not have hoped for a more favorable ruling by the Republican-dominated court. "It's a total victory," Minter said.

California already offers same-sex couples who register as domestic partners many of the legal rights and responsibilities afforded to married couples, including the right to divorce and to sue for child support.

Citing a 1948 California Supreme Court decision that overturned a ban on interracial marriages, the justices struck down the state's 1977 one-man, one-woman marriage law, as well as a similar, voter-approved law that passed with 61 percent in 2000.

The chief justice was joined by Justices Joyce Kennard and Kathryn Werdegar, all three of whom were appointed by Republican governors, and Justice Carlos Moreno, the only member of the court appointed by a Democrat.

In a dissent, Justice Marvin Baxter agreed with many arguments of the majority but said that the court overstepped its authority and that changes to marriage laws should be decided by the voters. Justices Ming Chin and Carol Corrigan also dissented.

California's secretary of state is expected to rule by the end of June whether the sponsors gathered enough signatures to put the gay-marriage amendment on the ballot.

Republican Gov. Arnold Schwarzenegger, who has twice vetoed legislation that would have granted marriage to same-sex couples, said in a statement that he respected the court's decision and "will not support an amendment to the constitution that would overturn this state Supreme Court ruling."

---

Associated Press writers Terence Chea, Jason Dearen, Juliana Barbassa and Evelyn Nieves in San Francisco and Liz Sidoti in Washington contributed to this report.