California's battle over gay marriage shifted from the ballot box to the courts Wednesday as stunned opponents of Proposition 8 declared they had too much at stake to concede.

Advocates of the measure to end same-sex marriage were certain they had succeeded in amending the state constitution to declare that "only marriage between a man and a woman is valid and recognized in California." But hours after they declared victory, the ACLU and a coalition of gay rights groups challenged the measure in the state Supreme Court, a move one backer called an "insult to California voters."

Opponents charged in their court petition the measure is invalid because it would undo the state constitution's commitment to equality for everyone.

The suit also contends the initiative process was improperly used because only the Legislature can place a measure before voters to so radically revise the state constitution.

Jenny Pizer, an attorney with the gay rights group Lambda Legal, said that allowing voters to remove a protected right conflicts with the constitution's basic ideals of equality.

"That's too big a change in the principles of our constitution to be made just by a bare majority of voters," Pizer said in a statement.

In May, the state Supreme Court ruled that a measure voters approved in 2000 violated the state constitution's clause on equal protection. Since then, more than 17,000 gay couples have been married, and weddings continued Wednesday.

Unlike the 2000 measure, Proposition 8 is a constitutional amendment that supporters are confident will withstand legal challenges.

Andrew Pugno, an attorney for the "Yes on 8" campaign, called Wednesday's lawsuit "frivolous" and "completely lacking in merit."

"Their position is absurd, an insult to California voters and an attack on the initiative process itself," Pugno said in a statement.

Mathew Staver, founder of Liberty Counsel and dean of the Lynchburg, Va.-based Liberty University School of Law, noted the court found earlier this year "there was no merit" in the revision argument.

"This is a constitutional amendment. It was on the ballot rightfully. (Opponents) have to accept the will of the people. They might not like it, but that's our democratic process," Staver said.

He said his group believes gay marriages in California this year are not valid but that it has no plans "to go out and roam the countryside trying to invalidate" them.

Democratic state Attorney General Jerry Brown told reporters his office would defend marriages "contracted during the time when the right to same-sex marriage was the law of California."

He said same-sex marriages enacted in recent months were not only lawful "but a fundamental constitutional right."

"The measure that was written by the Yes on 8 people was very simple, one sentence. It did not say the measure was retroactive," he said. He said civil measures must state specifically they are retroactive in order to be so.

Yet Brown said his office would also defend the "will of the people" in Tuesday's election against challenges to its legality.

"As Proposition 8 is challenged … we will defend the law as enacted by the people of California," he said.

Wednesday's suit was filed nine hours after supporters declared victory. With all precincts reporting, the measure was leading 52.5 percent to 47.5 percent, a difference of about 400,000 votes.

As a watershed election – Massachusetts and Connecticut are the only two other states where same-sex marriage is legal – the campaign generated more than $73 million in contributions.

Supporters of the measure said their side also benefited from a get-out-the-vote effort that included 100,000 volunteers.

Campaign officials said they were able to tap into higher-than-expected support from black voters, who were drawn to the polls by presidential candidate Barack Obama, and from Latino voters.

The apparent approval of the measure was a crushing blow for gay rights activists, who had hoped public opinion had shifted enough to help them block it.

Call Aurelio Rojas, Bee Capitol Bureau, (916) 326-5545. Bee Political Editor Amy Chance and The Bee's Andy Furillo contributed to this report.