Around 175 people were arrested in San Francisco while taking part in peaceful protests against a California Supreme Court decision to uphold a ban on same-sex marriage, police said.
A spokeswoman for the San Francisco Police Department said the arrests came as demonstrators blocked an intersection near the court building.
Those arrested were released at the scene, Sergeant Lyn Tomioka told AFP.
California's Supreme Court on Tuesday upheld a referendum that outlawed gay marriage, but said 18,000 same-sex weddings carried out before the ban would remain valid.
Gay and lesbian activists had sought to overturn the result of a November referendum, known as Proposition 8, which redefined marriage in California as being unions between men and women only.
However California Supreme Court justices said in a six to one majority opinion that the referendum -- which passed by a margin of 52.5 to 47.5 percent -- was legal and should be allowed to remain.
The decision came as a bitter blow for same-sex marriage advocates, who have recently celebrated notable victories elsewhere in the United States.
"There's no way to sugarcoat it; this is a very sad day for our community," said Shannon Minter, legal director for the National Center for Lesbian Right and an attorney who argued in court for the referendum to be overturned.
Minter said activists would now seek to place the issue back before voters.
"Today's decision is a terrible blow to same-sex couples ... But our path ahead is now clear. We will go back to the ballot box and we will win."
Crowds had gathered outside the California Supreme Court building more than an hour before the ruling and later faced off with police.
One same-sex marriage opponent carried a sign proclaiming: "Gay = Pervert."
After the ruling was released, gay marriage supporters chanted: "Shame on you, shame on you" as word quickly spread to the city's famous Castro neighborhood, immortalized in the recent Oscar-winning film "Milk."
"We're sad and upset," said Randy Nadeau, 43, who married William Lawson, 47, in California just two days before the referendum was approved by voters.
"I think after people get out of work at five o'clock things will get wild in the Castro. I'm in the mood for a little civil disobedience."
Robin Tyler, a plaintiff in one of the lawsuits challenging Proposition 8, said she expected thousands to protest in Los Angeles's gay hub of West Hollywood later Tuesday.
"There will be tens of thousands of us pouring onto the streets really angry, because for the first time in American history, gay people got into a constitution, and then people voted us out," Tyler said.
California Governor Arnold Schwarzenegger meanwhile urged protestors who take to the streets later Tuesday to respond "peacefully and lawfully."
"While I believe that one day either the people or courts will recognize gay marriage ... I will uphold the decision of the California Supreme Court," said state Governor Arnold Schwarzenegger.
While gay marriage supporters condemned the ruling, opponents of same-sex marriage applauded it as a righteous affirmation of traditional families.
"Today's decision is a victory for democracy and a victory for the civil rights of clergy, county clerks and Californians across the political spectrum who did not want to be forced by the government to approve of same-sex marriage," said Brad Dacus, president of the Pacific Justice Institute, a conservative group that supported Proposition 8.
Maine, Connecticut, Massachusetts, Vermont, and Iowa have all extended full marriage rights to gay couples, while New Hampshire and New York have edged closer to adopting such a law.
The latest ruling in California comes after a rollercoaster 12 months that has seen activists on both sides of the same-sex marriage debate lurch from jubilation to despair.
In May last year, California's Supreme Court voted four to three to legalize same-sex marriage, delighting gay activists in the most populous US state and sending thousands of same-sex couples rushing to tie the knot.
However the subject was forced back onto the political agenda by religious and social conservative groups, who gathered enough support for the issue to be put before voters at November 4 polls.
Rights activists swiftly challenged the legality of the referendum, arguing it was a revision of the state constitution that required a two-third vote in the legislature.