But opponents vowed to work even harder to convince voters this November to change the state’s constitution to ban same-sex marriage, and said they will work to postpone the weight of the decision until after the ballots are cast.

The ruling makes California the second state, after Massachusetts, to legally recognize gay marriage.

“I think it’s demonstrating that inch by inch we’re progressing and people aren’t nearly as fearful as they once were about what that means, gay marriage,” said Tracy’s Jack Elliott, 54, an openly gay ordained minister, human resources consultant and local business owner.

The ruling, assuming it’s not overridden by voters, opens up new possibilities previously reserved for heterosexual couples.

“I’m really happy that everyone who wanted to get married can now do it,” said Daniel Hadley, 19, an openly gay West High graduate and Las Positas college student.

The California ruling is considered monumental by virtue of the state's size — California counts 38 million residents 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," said Glen Lavy, senior counsel for the organization. "The remedy is a constitutional amendment. The constitution defines marriage as a union between one man and one woman."

Randy Thomasson of VoteYesMarriage.com, a Web site of 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.

But West High graduate Haldey said that those fighting against gay marriage “are starting to realize, that one, we’re not going to give up; and two, you’re fighting for an unjust cause."

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.

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.

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

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.

A spokesman for Republican Sen. John McCain, who opposes gay marriage, said the Arizona senator "doesn't believe judges should be making these decisions."

The campaigns of Sens. Barack Obama and Hillary Rodham Clinton said they believe that the issue of marriage should be left to the states.

• The Associated Press contributed to this report.