"(The ruling) is not the way a democracy is supposed to handle these sorts of heartfelt, divisive issues," said Brian Brown of the National Organization for Marriage, one of the groups helping to underwrite the gay marriage ban campaign. "I do think it will activate and energize Californians. I'm more confident than ever that we will be able to pass this amendment come November."

Twenty-six states have approved constitutional amendments banning same-sex marriage.

In the past few years, courts in New York, Maryland and Washington state have refused to allow gay marriage, and New Jersey's highest court gave the state lawmakers the option of establishing civil unions as an alternative.

Massachusetts is the only other state to legalize gay marriage, something it did in 2004. More than 9,500 same-sex couples in that state have wed. The California ruling is considered monumental because of the state's population — 38 million out of a U.S. population of 302 million — and its historical role as the vanguard of many social and cultural changes that have swept the country since World War II.

California has an estimated 108,734 same-sex households, according to 2006 census figures.

"It's about human dignity. It's about human rights. It's about time in California," San Francisco Mayor Gavin Newsom told a roaring crowd at City Hall after the ruling was issued. "As California goes, so goes the rest of the nation. It's inevitable. This door's wide open now. It's going to happen, whether you like it or not."

The case was set in motion in 2004 when Newsom threw open City Hall to gay couples to get married in a calculated challenge to California law. Four thousand wed before the Supreme Court put a halt to the practice after a month.

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

Gareth Lacy, a spokesman for Attorney General Jerry Brown, whose office argued to uphold the ban, said Brown would "work with the governor and other state agencies to implement the ruling."

The justices said they would direct state officials "to take all actions necessary to effectuate our ruling," including requiring county marriage clerks to carry out their duties "in a manner consistent with the decision of this court."

By Thursday afternoon, gay and lesbian couples had already started lining up at San Francisco City Hall to make appointments to get marriage licenses. The county clerk's office in Los Angeles issued a statement saying it was awaiting legal analysis of the ruling and a timeline for implementation.

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

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

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