Backers of Proposition 8 expressed anger at Brown's decision not to honor the will of voters, who approved the measure in November. "It's outrageous,"said Frank Schubert, campaign manager for Proposition 8.

But after studying the matter, Brown concluded that "Proposition 8 must be invalidated because the amendment process cannot be used to extinguish fundamental constitutional rights without compelling justification."

It is the attorney general's duty to defend the state's laws, and after gay rights activists filed legal challenges to Proposition 8, which amended the Constitution to ban same-sex marriage, Brown said he planned to defend the proposition as enacted by the people of California.

In a surprise move, state Atty. Gen. Jerry Brown asked the California Supreme Court on Friday to invalidate Proposition 8. He said the November ballot measure that banned gay marriage "deprives people of the right to marry, an aspect of liberty that the Supreme Court has concluded is guaranteed by the California Constitution."

Proposition 8 foes, however, were elated. "Atty. Gen. Brown's position that Proposition 8 should be invalidated demonstrates that he is a leader of courage and conviction," said Geoff Kors, executive director of Equality California.

In his brief to the high court, Brown noted that the California Constitution says that "all people are by nature free and independent and have inalienable rights," which include a right to "privacy."

The courts have previously said the right of a person to marry is protected as one of those inalienable rights, Brown wrote. The question at the center of the gay marriage cases, he told the justices, "is whether rights secured under the state Constitution's safeguard of liberty as an 'inalienable' right may intentionally be withdrawn from a class of persons by an initiative amendment." That, he concluded, should not be allowed.

Although voters are allowed to amend other parts of the Constitution by majority vote, to use the ballot box to take away an "inalienable" right would establish a "tyranny of the majority," which the Constitution was designed, in part, to prevent, he wrote. "For we are talking, necessarily, about rights of individuals or groups against the larger community, and against the majority -- even an overwhelming majority -- of the society as a whole."

The briefs filed Friday were in response to a spate of legal challenges filed by gay rights advocates, including the cities of San Francisco and Los Angeles.

Last month, the California Supreme Court announced that it would hear arguments in the case, perhaps as soon as March. A revision of the state Constitution can go before voters only after a two-thirds vote of the Legislature or a constitutional convention. Proposition 8 was put on the ballot after a signature drive. Brown's brief also said he believes that the estimated 18,000 same-sex marriages performed from June to November should remain valid.

Because it did not trust Brown to mount a staunch defense of the proposition, the group Protect Marriage intervened in the case and filed its own brief. It argued that Proposition 8 should remain legal and that the same-sex marriages performed from June to November should no longer be recognized.

--Jessica Garrison

Photo: Los Angeles Times

Updated and edited at 6:50 p.m.