"In contrast to earlier times, our state now recognizes that an individual's capacity to establish a loving and long-term committed relationship with another person and responsibly to care for and raise children does not depend upon the individual's sexual orientation," Chief Justice Ronald George wrote for the majority in ringing language that delighted gay rights activists.

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.

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.