Gay and lesbian couples will be able to marry again in California after the US Supreme Court's Prop 8 ruling Wednesday -- although it could take a few weeks, and opponents have vowed to fight on.

Same-sex couples celebrated the ruling in the famously liberal western US state, which briefly allowed gay marriages in 2008 before a close-run referendum decision outlawed them.

"I'm always the optimist and I think it's a great day," said Diane Olsen, half of the first lesbian couple to wed in Los Angeles in the half-year window when it was legal.

"This is the beginning of the end of discrimination," 59-year-old Olsen said, kissing her 71-year-old partner Robin Tyler on the lips at a press conference in Los Angeles.

The Supreme Court had been asked to rule on the constitutionality of California's Proposition 8, a 2008 ballot initiative that saw the nation's most populous state ban same-sex marriage by 52 for to 48 percent against.

Its decision instructed the 9th Circuit Court of Appeal in San Francisco to lift its stay on a 2010 ruling that found Prop 8 unconstitutional.

Within hours of the ruling in Washington DC, California's attorney general Kamala Harris called on the federal appeals court to lift the stay "immediately."

In theory the Supreme Court ruling will not go into effect for 25 days -- on Sunday, July 21.

But said Harris: "I'm .. calling on the 9th Circuit today to urge them in the strongest terms that they lift the stay," adding: "Same-sex couples have been denied equal protection under the law for far too long.

"Each one of those days that has passed has been a day that a family member who may have enjoyed and participated in a wedding ceremony may have passed away," she told reporters.

But while the ban is likely to be lifted within weeks, proponents of Prop 8 vowed Wednesday to pursue their efforts to have the ban -- backed by 52 percent of Californians five years ago, with 48 percent against -- re-enforced.

"We are pleased that the Supreme Court has reversed the Ninth Circuit Court of Appeals' misguided decision that sought to invalidate Proposition 8," said Andy Pugno, lawyer for

"For the more than 7 million Californians who have seen their vote stripped away from them, little by little, over the course of five years, that decision is gratifying," he said, referring to the Californian voters who backed Prop 8.

"We will continue to defend Prop 8 and seek its enforcement until such time as there is a binding statewide order that renders Prop 8 unenforceable," he added.

Law professor John Eastman of California's Chapman University said Prop 8 supporters could mount a challenge under a state constitutional provision that stops officials refusing to follow laws without an order from an appeals court.

"This is far from over, I can tell you," Eastman, who supports the same-sex marriage ban, told the LA Times.

But Santa Clara University law professor Gerald Uelmen poured scorn on that analysis. "I think they would be laughed out of state court," he told the newspaper.