SAN FRANCISCO - California's Supreme Court upheld the state's gay-marriage ban yesterday but said the 18,000 same-sex weddings that took place before the prohibition passed were still valid - a ruling decried by gay-rights activists as a hollow victory.

Demonstrators outside the court booed, wept, and yelled, "Shame on you!" Activists said they would go back to the voters as early as next year in a bid to repeal the ban.

In a 6-1 decision written by Chief Justice Ron George, the court rejected arguments that the ban approved by voters last fall was such a fundamental change in the California Constitution that it first needed the Legislature's approval.

"We are extremely pleased that the Supreme Court has acknowledged the right of voters to define marriage in the California Constitution," said Andrew P. Pugno, a lawyer for ProtectMarriage.com, the leading group behind the initiative. "The voters have decided this issue and their views should be respected."

As for the thousands of couples who tied the knot last year in the five months that gay marriage was legal in California, the court said it was a well-established principle that an amendment is not retroactive unless it is clear that the voters intended it to be, and that was not the case with Proposition 8.

Moreover, the court said it would be too disruptive to apply Proposition 8 retroactively and dissolve all gay marriages.

Doing that would have the effect of "throwing property rights into disarray, destroying the legal interests and expectations of thousands of couples and their families, and potentially undermining the ability of citizens to plan their lives according to the law as it has been determined by this state's highest court," the ruling said.

While gay-rights advocates accused the court of failing to protect a minority group from the will of the majority, the justices said that the state's governing framework gave voters almost unfettered ability to change the California Constitution.

The decision set off an outcry among demonstrators who had gathered in front of the San Francisco courthouse, holding signs and waving rainbow flags. Many people also held hands in a chain around an intersection in an act of protest.

In San Francisco's Castro district, where many gay men and lesbians live, the large rainbow gay-pride flag that flies in the neighborhood's Harvey Milk Plaza had been lowered to half-staff and a black stripe put on the top.

"We're relieved our marriage was not invalidated, but this is a hollow victory because there are so many that are not allowed to marry those they love," said Amber Weiss, 32, who was in the crowd at City Hall with her partner, Sharon Papo.

"I feel very uncomfortable being in a special class of citizens," Papo said.

The state Supreme Court ruled 4-3 last May that it was unconstitutional to deny gay couples the right to wed. In what gay activists called their "Summer of Love," same-sex couples from around the country rushed to get married in California for fear the voters would take away the right at the ballot box. In November, Proposition 8 passed with 52 percent approval.

Gay-rights activists argued that the ban was improperly put to the voters and amounted to a revision - which required legislative approval - not an amendment. But the justices disagreed.

The court said that while the ban denies gay couples use of the term marriage, it does not disturb their basic right to "establish an officially recognized and protected family relationship with the person of one's choice." California still allows gay couples to form domestic partnerships.

Justice Carlos Moreno, the lone dissenter, said that denying same-sex couples the right to wed "strikes at the core of the promise of equality that underlies our California Constitution."

Before last fall, California and Massachusetts were the only states to allow same-sex marriage. Iowa, Connecticut, Vermont, and Maine have since legalized it, and lawmakers in New York, New Jersey, and New Hampshire are considering bills of their own.