In what could be a major setback for gay rights in the US, California's Supreme Court has invalidated nearly 4000 marriage licences issued to same-sex couples in San Francisco this year.

In its highly anticipated ruling, the court ruled that San Francisco officials overstepped their authority and misinterpreted state laws by granting marriage licences to homosexual couples who flocked to the city in February and March.

The court did not resolve whether the California Constitution permits same-sex marriages, and gay rights advocates have filed a separate suit seeking a ruling.

"Should the current California statutes limiting marriage to a man and a woman ultimately be repealed or be held unconstitutional, the affected couples then would be free to obtain lawfully authorised marriage licences," the court noted.

Thursday's ruling pertains only to California, but gay marriage opponents said it would undoubtedly have an effect elsewhere.

"It bodes well for any us trying to protect the institution of marriage," said Christian Coalition of Georgia chairman Sadie Fields.

Fields' group is behind a referendum that will ask Georgia voters to decide on November 2 whether to amend the state constitution to ban same-sex marriages and limit other rights for homosexuals.

At the Alliance Defence Fund, the conservative Arizona-based group that sued to nullify the San Francisco marriage licences, chief counsel Benjamin Bull called the ruling "a huge momentum-killer" for gay rights activists.

"San Francisco is the epicentre of gay rights activity," he said. "They've lost on their home turf."

But Thursday's biggest losers were the 3995 gay couples who thought they were legally married and could be entitled to the same spousal rights as heterosexual married couples. The court's vote to nullify those unions was 5-2.

During the four weeks San Francisco issued the marriage licences, many who came to the city to get hitched compared it to a civil rights movement and an opportunity they never had before.

In a statement, the first same-sex couple to receive a licence, Del Martin and Phyllis Lyon, said Thursday's ruling had crushed them.

"Del is 83 years old and I am 79," said Ms Lyon. "After being together for more than 50 years, it is a terrible blow to have the rights and protections of marriage taken away from us."

Without a marriage licence, a surviving partner can make no claim to a dead partner's social security, pension or other benefits, according to the Lambda Legal Defence and Education Fund, which fought the lawsuit that led to Thursday's ruling.

- Cox