California's Supreme Court upheld the state's same-sex marriage ban on Tuesday, but ruled the roughly 18,000 existing same-sex marriages carried out in the state before the ban can stand.

Frank Capley-Alfano is hugged by gay rights advocate Davina Kotulski after a ruling by the California Supreme Court on gay marriage in San Francisco on Tuesday. Capley-Alfano and Kotulski both are married to same-sex partners. (Marcio Jose Sanchez/Associated Press)The ban, known as Proposition 8 became law in the state after California voters passed the amendment in a Nov. 4, 2008, referendum.

A large crowd of protesters outside the San Francisco courthouse booed and chanted, "shame on you" when the 6-1 decision was announced.

In the court's majority ruling, the justices said it not their job to address whether the ban is wise public policy, but to decide whether it is constitutionally valid.

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."

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

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

Plan for new vote in 2010

Supporters of same-sex marriage have already said they want to put another proposition over the issue on the 2010 state ballot, while Tuesday's decision could lead to another legal challenge before the top court in the United States, the Supreme Court in Washington.

"It's not about whether we get to stay married. Our fight is far from over," said Jeannie Rizzo, 62, who was one of the lead plaintiffs along with her wife, Polly Cooper.

"I have about 20 years left on this earth, and I'm going to continue to fight for equality every day."

Proposition 8 was the subject of a heated, costly battle ahead of election day and passed with 52 per cent of the vote. It overruled a California Supreme Court decision of May 2007 that made marriage between two members of the same sex legal.

Gay-rights legal groups filed a petition asking the Supreme Court to invalidate Proposition 8 on the grounds that voters did not have the authority to make such a dramatic change in state law without approval from the legislature.

The state Supreme Court decided to hear three legal challenges on the ban, but refused to allow gay couples to resume marrying until it ruled.

Thirty states in the U.S. have banned same-sex marriage, although several permit civil unions or domestic partnerships that recognize some marriage rights.

States that currently allow same-sex marriage are Massachusetts, Connecticut and Iowa. Vermont and Maine have passed legislation to recognize same-sex marriages in their states later in the year.

With files from The Associated Press