SAN FRANCISCO, Aug. 12 - The California Supreme Court ruled Thursday that a blitz of same-sex weddings here in February and March had no standing under state law and declared the marriages "void and of no legal effect from their inception."

In a unanimous ruling, the justices said that Mayor Gavin Newsom of San Francisco had exceeded his authority in allowing the marriages. By a 5-to-2 vote, the court also rendered the licenses, more than 4,000 in all, nothing more than a collector's item, ordering city officials to remove any record of them from the books and to offer the couples refunds of license fees.

The decision was narrow legally, applying only to the issue of executive authority and not to the constitutionality of state law limiting marriage to unions between a man and a woman.

Yet in practical terms, though previous rulings here and elsewhere in the country have placed thousands of same-sex marriages in limbo, the decision on Thursday was the biggest and most definitive reversal of gay weddings anywhere because of the nullification of the marriages.

"Withholding or delaying a ruling on the current validity of the existing same-sex marriages might lead numerous persons to make fundamental changes in their lives or otherwise proceed on the basis of erroneous expectations, creating potentially irreparable harm," Chief Justice Ronald George wrote for the majority.

In a dissenting opinion, Justice Joyce L. Kennard said it was too early to throw out the licenses because of a separate constitutional challenge now in San Francisco Superior Court to the prohibition on gay marriage in California state law.

"It is premature and unwise to assert, as the majority essentially does, that the thousands of same-sex weddings performed in San Francisco were empty and meaningless ceremonies in the eyes of the law," Justice Kennard wrote.

In issuing the licenses between Feb. 12 and March 11, when the Supreme Court ordered a halt to them, city officials warned the same-sex couples that the marriages had uncertain legal standing. But in the general euphoria of the time, few of the couples cared much about the caveat. In the early days before an appointment system was put in place, thousands of people lined up at City Hall in the hope of getting married.

Some of the same-sex married couples said they later used the licenses to obtain benefits previously denied them, like discounts on insurance.

Dennis Herrera, the San Francisco city attorney, said Thursday that he could offer no guidance to couples who had received such benefits, suggesting only that they consult a lawyer. Molly McKay, a lawyer who is associate executive director of Marriage Equality California, a gay rights group, said her own auto insurance discount of $316 might now be in jeopardy.

"Every year we called them to determine whether we qualify for the marital discount as domestic partners," Ms. McKay said of her insurance company, "and every year they said no." But after Ms. McKay and her partner of seven years got married, she said, "We called our agent and said: Guess what? We're married. Do we get the discount? And they said sure."

Ms. McKay, who showed up outside the courthouse on Thursday and wore her wedding dress, and the leaders of other gay and lesbian groups said the ruling had unleashed a torrent of questions from scores of suddenly unmarried same-sex couples. The National Center for Lesbian Rights posted a series of "Frequently Asked Questions" on its Web site to help address some of the concerns.

"The interesting question is, what happens next?" Ms. McKay said. "Do we file a joint tax return for the six months of our marriage and then file another return for the rest of the year separately as single?"

Yet even with the many pragmatic concerns, the most prevalent reactions to the ruling among gay-marriage advocates were anger and sadness. Several gay couples gathered on the steps outside the Supreme Court here to wait for the ruling and wept as excerpts from the court's 114-page decision were read aloud.

"I feel more pain than I expected to," said Kate Kendell, a lawyer who is the executive director of the National Center for Lesbian Rights and who acted as a witness at the first gay wedding on Feb. 12. "My lawyer brain is being seeped into by my human brain."

The decision was hailed by conservative groups opposed to same-sex marriage as an important repudiation of San Francisco's bid to skirt a state law, approved by voters in 2000, that defines marriage as "a personal relation arising out of a civil contract between a man and a woman."

Jordan Lorence, a lawyer for the Alliance Defense Fund, a Christian advocacy group that sued the city over the same-sex marriages, said the decision amounted to a "huge message" to Mr. Newsom and other gay-marriage advocates that they cannot break the law.

In issuing their ruling, the Supreme Court justices said Mr. Newsom, who had argued the state law on marriage was unconstitutional, had invited chaos by overstepping his bounds as mayor.

"This conclusion is consistent with the classic understanding of the separation of powers doctrine - that the legislative power is the power to enact statues, the executive power is the power to execute or enforce statutes, and the judicial power is the power to interpret statutes and to determine their constitutionality," Chief Justice George wrote.

"There are thousands of elected and appointed public officials in California's 58 counties charged with the ministerial duty of enforcing thousands of state statutes," he wrote later. "If each official were empowered to decide whether or not to carry out each ministerial act based upon the official's own personal judgment of the constitutionality of an underlying statute, the enforcement of statutes would become haphazard, leading to confusion and chaos."

Hailing the ruling, Mr. Lorence said, "Change comes by respecting regular procedures, not by defying state law."

He and others who had challenged the marriages gathered across the street from the Supreme Court building.

Another lawyer for the group, Joshua Carden, said the court had rendered the San Francisco marriages meaningless.

"It's like counterfeit money," Mr. Carden said of the licenses, "you can't spend it and you can't cash it."

Mr. Newsom, speaking at a news conference at City Hall, said he would respect the ruling, though he said he disagreed with it. He offered no apologies for his actions, except to say that his bid to bring same-sex marriage to California would take longer than he had hoped.

"I think what we did was right and appropriate and history will judge that," Mr. Newsom said.

As satisfying as the ruling was for the opponents of gay marriage, many agreed that the biggest fight was yet to come. In issuing its ruling, the Supreme Court expressly sidestepped the fundamental question of whether the state's ban on same-sex marriage was constitutional. That question is the subject of a separate lawsuit that could take several years before reaching the State Supreme Court.

With that fight in mind, gay-marriage advocates organized a protest march late on Thursday, beginning in the heavily gay Castro district and ending at Civic Center Plaza near the Supreme Court.

Mr. Newsom said he was convinced that gay-marriage proponents would achieve victory in his lifetime and in the lifetimes of the first couple to be married at City Hall, Del Martin, 83, and Phyllis Lyon, 79.

Mr. Lorence, the lawyer for the Alliance Defense Fund, disagreed. "People will step in," he said. "Previous political actions in various states have shown that the voters will step in when the states get it wrong."

Carolyn Marshall contributed reporting for this article.