Chief US District Judge Vaughn Walker's strongly worded opinion in the landmark case — the first in a federal court to examine if states can lawfully limit marriage to a man and a woman — touched off a celebration outside the courthouse.
Later in the day, a jubilant crowd marched through the city that has long been a haven for gays.
As word of the verdict spread, about 300 people assembled in a West Hollywood park waving rainbow gay pride flags. In New York City, a crowd of about 150 gathered outside a lower Manhattan courthouse. They carried signs saying "Our Love Wins" as organisers read portions of the 136-page decision aloud.
Ruling in a lawsuit filed by two couples who claimed the ban violated their civil rights, Judge Walker methodically dismantled every argument advanced by sponsors of the voter-approved measure, known as Proposition 8, before declaring it unconstitutional.
"Proposition 8 singles out gays and lesbians and legitimates their unequal treatment," he wrote. "Proposition 8 perpetuates the stereotype that gays and lesbians are incapable of forming long-term loving relationships and that gays and lesbians are not good parents."
Standing in front of eight American flags at a news conference, the two couples behind the case beamed and choked up as they related their feelings of validation.
"Our courts are supposed to protect our Constitutional rights," lead plaintiff Kris Perry said as Sandy Stier, her partner of 10 years, stood at her side. "Today, they did."
"We are not here to change the world. Equality is something our country has always been about," said co-plaintiff Jeff Zarrillo, who is seeking the right to marry his partner of nine years, Paul Katami. "Today's decision brings Paul and I and so many others like us closer to that equality, too."
Voters passed Proposition 8 as a state constitutional amendment in November 2008, five months after the California Supreme Court legalised same-sex unions and an estimated 18,000 same-sex couples already had tied the knot. Despite yesterday's ruling, it remains uncertain when gay weddings will be allowed to resume in the state.
Protect Marriage, the coalition of religious and conservative groups that sponsored the ban, said it would appeal the ruling to the 9th US Circuit Court of Appeals.
"In America, we should uphold and respect the right of people to make policy changes through the democratic process, especially changes that do nothing more than uphold the definition of marriage that has existed since the founding of this country and beyond," said Jim Campbell, a lawyer on the defense team.
Judge Walker, meanwhile, said he would consider while Protect Marriage pursues its appeal suspending an order requiring the state to cease enforcing Proposition 8. He ordered both sides to submit written arguments by tomorrow on the issue.
The appeal would go first to the 9th Circuit, then to the US Supreme Court, if the high court justices agree to review it.
Currently, same-sex couples can legally wed only in Massachusetts, Iowa, Connecticut, Vermont, New Hampshire and Washington, DC
Before deciding the case, Judge Walker heard 13 days of testimony and arguments. Former US solicitor General Theodore Olson teamed with David Boies to argue for the plaintiffs, bringing together two litigators best known as adversaries who respectively represented George W Bush and Al Gore in the disputed 2000 election.
Reveling in their joint victory Wednesday, Mr Boies said their unusual alliance would be valuable if the Proposition 8 case, known as Perry v. Schwarzenegger, reaches the Supreme Court.
"Ted and I have a deal: He is going to get the five justices that were for him in Bush v Gore, and I'm going to get the four justices that were with me in Bush v Gore," Mr Boies joked.
Defense lawyers argued at trial that the ban was necessary to safeguard the traditional understanding of marriage and to encourage responsible childbearing. But they called just two witnesses, claiming they did not need to present expert testimony because the US Supreme Court had never specifically recognised a right to gay marriage.
"To prevail in the end, our opponents have a very difficult task of convincing the US Supreme Court to abandon precedent and invent a new constitutional right," Andy Pugno, a lawyer for Protect Marriage said.
The judge, however, dismissed the notion that gay Americans were seeking a new right as opposed to one already guaranteed them as US citizens under the Constitution's due process and equal protection clauses.
Preventing gays from marrying does nothing to strengthen heterosexual unions or serve any purpose that justifies the ban's discriminatory effect, he said.
"Same-sex couples are identical to opposite-sex couples in the characteristics relevant to the ability to form successful marital unions," the judge wrote. "Like opposite-sex couples, same-sex couples have happy, satisfying relationships and form deep emotional bonds and strong commitments to their partners."
"Indeed, the evidence shows Proposition 8 does nothing more than enshrine in the California Constitution the notion that opposite-sex couples are superior to same-sex couples," the judge wrote.
Describing the defense case as "a rather limited factual presentation," he also said its proponents offered little evidence that they were motivated by anything other than animus toward gays — beginning with their campaign to pass the ban, which included claims of wanting to protect children from learning about same-sex marriage in school.
"Proposition 8 played on the fear that exposure to homosexuality would turn children into homosexuals and that parents should dread having children who are not heterosexual," he wrote.
The ruling puts the judge, a Republican appointed by president George H W Bush, at the forefront of the gay marriage debate and marks the second verdict in a federal gay marriage case to come down in less than a month.
In July, a District Court judge in Massachusetts decided the state's legally married gay and lesbian couples had been wrongly denied the federal financial benefits of marriage because of a law preventing the US government from recogniing same-sex unions.
SIC: IT