SAN FRANCISCO — In a sharp rebuke to supporters of a contested state ballot measure that banned same-sex marriage, the California attorney general said Friday that the measure was constitutionally indefensible and should be overturned.

The attorney general, Jerry Brown, had previously hinted of his opposition to the measure, Proposition 8, but made his legal opinion concrete Friday in a brief to the California Supreme Court, which is reviewing the measure.

“Proposition 8 must be invali-dated because the amendment process cannot be used to extinguish fundamental constitutional rights without compelling justification,” Brown said in a statement.

Opponents have argued that the proposition fundamentally altered the state constitution by taking away the right to marry from same-sex couples, who had been free to do so since May, when the California Supreme Court legalized such marriages. Proposition 8 overturned that decision by defining marriage in California as between only men and women.

Supporters of Proposition 8 asked the court in a separate legal brief filed Friday to invalidate the approximately 18,000 same-sex weddings performed before the ban was passed.

“The language, policy, history and intent of Proposition 8 do not permit recognition of some same-sex marriages but not others,” read the brief, which was co-written by the prominent conservative lawyer Kenneth Starr. “None are valid or recognized in California.”

Opponents of Proposition 8 said the brief was another effort to strip rights from same-sex couples who were already hurting from the measure’s passage.

“This is an offensive attempt to rip away people’s marriages,” said Geoff Kors, the executive director of Equality California, a gay-rights group. “There was nothing in Proposition 8 that was retroactive.”

Brown’s action did not surprise supporters of Proposition 8. “It’s no secret that the attorney general opposed Proposition 8 during the campaign,” said Andrew Pugno, the lawyer for the group behind the ban, Protect Marriage.