A United States group opposed to same-sex marriages has asked the California Supreme Court to annul thousands of gay marriages held in the state this year.
Thousands of same-sex couples wed following a ruling by the Supreme Court in June that such marriages were permissible.
But in a November referendum, held at the same time as the US presidential election, 52.1% of California voters backed Proposition 8, a measure to amend the state constitution to define marriage as between a man and a woman only.
A Proposition 8 campaign group, called Yes on 8, says the same-sex unions should not be recognised.
The group filed its court brief in response to several legal challenges seeking to overturn Proposition 8.
"Proposition 8's brevity is matched by its clarity," said the group's brief.
Supporters of same-sex marriages say the ban on such unions cannot be applied retroactively.
California's attorney general, Jerry Brown, also filed a brief on Friday, saying the Proposition 8 amendment was unconstitutional and deprived gay couples of a fundamental right, Associated Press news agency reported.
The Supreme Court has said it will hear the challenges to Proposition 8, but that pending its ruling, the ban on gay marriage remains in effect.
The court has said a hearing on the challenges could be held by next March.
Connecticut and Massachusetts are the only US states to allow gay marriage. Several other states allow civil unions.