Gay couples celebrated today after a judge overturned California's same-sex-marriage ban.

The ruling by Chief US District Judge Vaughn Walker could eventually force the US Supreme Court to confront the question of whether gay couples have a constitutional right to wed.

Outside the court in San Francisco, cheering gay couples waved rainbow and American flags.

Shelly Bailes embraced her wife Ellen Pontac as Ms Bailes held a sign reading "Life Feels Different When You're Married".

In New York City, about 150 people gathered in front of a lower Manhattan court, carrying signs saying "Our Love Wins" as organisers read portions of the ruling.

Judge Walker made his decision in a lawsuit filed by two gay couples who claimed the voter-approved ban violated their civil rights.

The ruling "vindicates the rights of a minority of our citizens to be treated with decency and respect and equality in our system," said former US solicitor general Theodore Olson, who delivered the closing argument at trial for opponents of the ban.

California governor Arnold Schwarzenegger also hailed the ruling as an important step towards equality and freedom.

But Protect Marriage, the coalition of religious and conservative groups that sponsored the ban, said it would immediately appeal against 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 defence team.

Despite the favourable ruling for same-sex couples, gay marriage will not be allowed to resume immediately.

Judge Walker said he wanted to decide whether his order should be suspended while the supporters of the ban pursue their appeal. 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.

California voters passed the ban, called Proposition 8, in November 2008, five months after the state supreme court legalised gay marriage.

Supporters argued the ban was necessary to safeguard the traditional understanding of marriage and encourage responsible childbearing.

Judge Walker, however, found it violated the state constitution's due process and equal protection clauses while failing "to advance any rational basis in singling out gay men and lesbians for denial of a marriage licence".

"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," he wrote in his 136-page ruling.

He also said proponents offered little evidence that they were motivated by anything other than animosity towards 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 a fear that exposure to homosexuality would turn children into homosexuals and that parents should dread having children who are not heterosexual," Judge Walker wrote.

The judge heard 13 days of evidence and arguments since January during the first trial in federal court to examine if states could prohibit gays from getting married.

Former US Justice Department lawyer Charles Cooper, who represented the religious and conservative groups that sponsored the ban, said cultures around the world, previous courts and the US Congress accepted the "common sense belief that children do best when they are raised by their own mother and father".

Currently, same-sex couples can only legally wed in Massachusetts, Iowa, Connecticut, Vermont, New Hampshire and Washington DC.

The ruling puts Judge Walker, a Republican, at the forefront of the gay marriage debate and marks the latest in a long line of high-profile legal decisions for the long-time federal justice.

He was appointed by Ronald Reagan, but his nomination was held up for two years in part because of opposition from gay rights activists.

As a lawyer, he helped the US Olympic Committee sue a gay ex-Olympian who had created an athletic competition called the Gay Olympics.