Massachusetts is the only other state to legalize gay marriage, something it did in 2004. The California ruling is considered monumental by virtue of the state's size — 38 million out of a U.S. population of 302 million — and its historic role in the vanguard of the many social and cultural changes that have swept the country since World War II.

California has an estimated 92,000 same-sex couples.

"It's about human dignity. It's about human rights. It's about time in California," San Francisco Mayor Gavin Newsom, pumping his fist in the air, told a roaring crowd at City Hall. "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."

Unlike Massachusetts, California has no residency requirement for obtaining a marriage license, meaning gays from around the country are likely to flock to the state to be wed, said Jennifer Pizer, a gay-rights attorney who worked on the case.

The ultimate reach of the ruling could be limited, however, since most states do not recognize gay marriages performed elsewhere. Nor does the federal government.

The conservative Alliance Defense Fund said it would ask the justices for a stay of the decision until after the fall election in hopes of adding California to the list of 26 states that have approved constitutional amendments banning same-sex marriage.

"We're obviously very disappointed in the decision. The remedy is a constitutional amendment. The constitution defines marriage as a union between one man and one woman," said Glen Lavy, senior counsel for the organization.

Randy Thomasson of VoteYesMarriage.com, a campaign to amend the California Constitution to ban gay marriage, said the decision was in effect telling children that they have a "new role model — homosexual marriage, aspire to it.

"This is a disaster," he said.

Opponents of gay marriage could also ask the high court to reconsider. If the court rejects such a request, same-sex couples could start getting married in 30 days, the time it typically takes for the justices' opinions to become final.

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 court's decision.

James Vaughn, director of the California Log Cabin Republicans, called the ruling a "conservative one."

"The justices have ensured that the law treats all Californians fairly and equally. This decision is a good one for all families, gay and non-gay," Vaughn said.

The case was set in motion in 2004 when the mayor of San Francisco — the unofficial capital of gay America — threw City Hall open to gay couples to get married in a calculated challenge to California law. Four-thousand gay couples wed before the Supreme Court put a halt to the practice after a month.