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

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

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

Supporters of Prop. 8 hailed the ruling as a victory for the will of the people.

"Any decision overturning Proposition 8 would have been a clear attack on fundamental democratic principles," said attorney Robert Tyler, who fought for Prop. 8 through his Murrieta-based firm Advocates for Faith and Freedom.

Andrew P. Pugno, a lawyer for ProtectMarriage.com, the leading group behind the initiative, said his group was pleased the state Supreme Court has acknowledged the right of voters to define marriage.

"The voters have decided this issue and their views should be respected," he said.

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 is 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 Prop. 8.

Moreover, the court said it would be too disruptive to apply Prop. 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 gives voters almost unfettered ability to change the California Constitution.

The decision set off an outcry among a sea of 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. More than 150 protesters were arrested, with citations for failure to obey a police officer and jaywalking.

About 80 protesters rallied outside the Los Angeles County clerk's office, where marriage licenses are issued. They waved rainbow flags and carried signs that read "Repeal Prop 8 in 2010."

In San Francisco's Castro district, where many gay men and lesbians live, the large rainbow gay pride flag that flies in 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, near the courthouse, with her wife, Sharon Papo. They were married on the first day that gay marriage was legal last year, June 17.

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

Jeanne Rizzo, 62, who was one of the plaintiffs along with her wife, Pali Cooper, said: "It's not about whether we get to stay married. Our fight is far from over. I have about 20 years left on this Earth, and I'm going to continue to fight for equality every day."

A San Francisco police officer said many of the officers staffing the protest were members of the department's group for gay, lesbian and transgender police.

"I go out every day fighting for people's lives, but I can't get married to who I want to," said Officer Lenny Groberg, 52, the first openly gay officer assigned to the city's gang task force.

A small group of Prop. 8 supporters also gathered outside the court.

"A lot of people just assume we're religious nuts. We're not. But we are Christians and we believe in the Bible," said George Popko, 22, a student at American River College in Sacramento, where the student government officially endorsed Prop. 8.

In the state capital, Republican state Assemblyman Sam Blakeslee of San Luis Obispo, the incoming minority leader, said the court's decision "reaffirmed the principle that the people's votes do matter."

Last May, the state Supreme Court ruled 4-3 that it was unconstitutional to deny gay couples the right to wed. For a while, that put California -- the nation's most populous state -- back in its familiar position in the vanguard of social change; at the time, Massachusetts was the only other state to allow gay marriage.

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, Prop. 8 passed with 52 percent approval.

As the fight went on in California, Iowa, Maine, Vermont and Connecticut legalized gay marriage, bringing to five the number of states that allow same-sex couples to wed.

In California, 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 fundamentally disturb their basic right to "establish an officially recognized and protected family relationship with the person of one's choice and to raise children within the family." California still allows gay couples to form domestic partnerships.

In their 136-page majority ruling, the justices said it was not their job to address whether the ban is wise public policy, but to decide whether it is constitutionally valid, while "setting aside our own personal beliefs and values."

Justice Carlos Moreno was the lone dissenter.

He said denying same-sex couples the right to wed "strikes at the core of the promise of equality that underlies our California Constitution." He said it represents a "drastic and far-reaching change."

"Promising equal treatment to some is fundamentally different from promising equal treatment for all," Moreno wrote. "Promising treatment that is almost equal is fundamentally different from ensuring truly equal treatment."

Gay rights supporters have planned a rally in Fresno this weekend to drum up support in the state's conservative-leaning center for a measure to repeal the ban either during next year's gubernatorial election or in 2012.

The battle also is being taken to federal court.

Prominent lawyers Theodore B. Olson and David Boies filed a U.S. District Court lawsuit Friday on behalf of two gay men and two gay women, arguing that Prop. 8 violates the U.S. constitutional guarantee of equal protection and due process.

Olson said he hopes the case, which seeks a preliminary injunction against the measure until the case is resolved, will wind up before the U.S. Supreme Court.