Thousands of gay couples are in legal limbo after voters in supposedly liberal California shocked pollsters and equality campaigners by voting to reverse same-sex marriage legislation.

In a massive setback for the gay rights movement, 52.5 per cent of the electorate backed Proposition 8, a ballot measure that adds to the state constitution the words: "only marriage between a man and a woman is valid or recognised in California."

The result sparked outrage among the 18,000 couples who have rushed to marry since the Supreme Court legalised same-sex weddings five months ago. Many joined noisy street protests in San Francisco and Los Angeles as the outcome of the vote became clear.

In West Hollywood, police and demonstrators clashed after roughly 500 protesters veered off the permitted route for their impromptu march. Arrests were made but officials said most of the 2,500-strong crowd had been largely law-abiding.

Ellen DeGeneres, the lesbian chat-show host who became a "face" of the gay rights movement when she married her partner, Portia de Rossi, said she was "saddened beyond belief" by the passage of the proposition.

Like any good, all-American controversy, the battle seems destined to end up in court. In three new lawsuits, newly-wed couples have argued that the way Proposition 8 was put to voters breached electoral protocol.

The state Supreme Court must now consider the gay rights lobby's claim that banning same-sex marriage is a constitutional "revision," rather than a more limited "amendment" and so the state legislature should have approved it before putting it to the electorate.

"I believe that it's a sound legal argument based on a deep appreciation of our constitution," said Jenny Pizer, a lawyer working on one challenge.

A spokesman for No on 8, the group co-ordinating the legal challenges, said: "We feel crushed and disappointed, but we are confident that right is on our side. This is a shameful day for California, which has voted against a fundamental civil right, but we are hopeful of winning it back in court."

In light of Barack Obama's victory in the presidential election, many of the plaintiffs attempted to link their struggle to the wider civil rights movement.

Gloria Allred, who filed one law suit yesterday, said: "This has been a heartbreaking experience for our clients, Robin Tyler and Diane Olson, and millions of other same-gender couples. All they have asked for is equal rights under the law and equal respect and dignity for their families and their committed relationships. Our law firm is honoured to continue this great civil rights battle for them. We will never give in and we will never give up."

The reference to civil rights felt ironic, since exit polls suggested that Proposition 8 had passed because of a higher-than-expected turnout from California's socially conservative black community – 70 per cent of black voters voted for a gay marriage ban, helping it achieve a 400,000-vote majority. White voters were marginally opposed to the ban. Hispanics were split.

The proposition's supporters had previously mounted a disciplined $40m campaign funded by members of religious groups, including the Catholic and Mormon churches.

Conservatives from every state in America flooded California in the run-up to the poll, focusing their campaign on the implications of gay marriage for schools, churches and children.

They convinced voters that gay weddings would force pastors to marry couples even if they didn't want to, that homosexuality would be taught in infant schools, and that family life would be undermined.

"Mommy, mommy, today I learnt that a Prince can marry a Prince and I can one day marry a princess," said a small girl in one of their attack ads.

The No on 8 campaign described its opponents' arguments as scare tactics but it was unable to convince voters, running vague TV ads which made scant use of potential resources, particularly those within the local celebrity community.

The new lawsuits were deemed undemocratic by opponents, who also succeeded in getting gay marriage bans into the constitutions of Florida and Arizona – a symbolic move, since neither state has legalised same-sex unions.

Tying the knot: How the same-sex marriage battle unfolded in California

February 2004 The San Francisco Mayor, Gavin Newsom, announces that the city will issue marriage licences to same-sex couples.

March 2004 Over 4,000 gay weddings are held in San Francisco, but California's Supreme Court issues an injunction against further ceremonies until it has reviewed their legality.

August 2004 After lengthy hearings, the court says Mr Newsom's unilateral decision to issue marriage licences to gays is illegal and the 4,000 same-sex unions should be declared void.

May 2008 An appeal against the 2004 decision, saying it violated the constitutional rights of same-sex couples and breached discrimination laws, is upheld by a 4-3 majority.

June 2008 The appeal ruling takes effect. More than 18,000 gay couples, including the Hollywood actresses Ellen DeGeneres and Portia de Rossi, pictured right, marry over the next five months.

November 2008 Proposition 8, saying "only marriage between man and woman is valid or recognised in California", is passed by 52.5 per cent of voters.