SAN FRANCISCO (Reuters) - California's attorney general predicted on Monday that the courts would bar same-sex marriages in San Francisco within weeks and invalidate the thousands of gay marriages the city has sanctioned over the past two weeks.

"I think it will be weeks and my best prediction is that the marriages will be invalidated, the courts will direct people that wish to change the law to the legislative process," Bill Lockyer, the Democratic attorney general, told Reuters in a telephone interview.

Over the past 10 days, San Francisco has issued marriage licenses to 3,175 homosexual couples, prompting euphoria in the gay community and criticism from backers of traditional male-female nuptials.

California Gov. Arnold Schwarzenegger has criticized the marriages and asked Lockyer to resolve the issue immediately.

"I believe strongly in the law that we have right now in California which respects domestic partnership rights and I think that that's a very good law," the former action film star said in Washington. "I believe in equal rights absolutely and in protecting that."

Lockyer was expected to present legal arguments against the same-sex marriages within a day or two. San Francisco has sued California, saying the state's marriage laws violate the state constitution's provisions on equality.

"My duty is very clear. It is to defend the state law," said Lockyer, a possible rival to Republican Schwarzenegger in the 2006 gubernatorial election. "We're going to respond to their lawsuit which they filed late last week."

POLL FINDS HALF OF STATE OPPOSES GAY MARRIAGE

California law, backed most recently by a voter initiative in 2000, defines marriage as a union of man and woman. A Public Policy Institute of California poll of 2,004 Californian adults released on Friday found that exactly half opposed gay marriage and 44 percent approved.

At San Francisco City Hall, which had become a scene of celebration since newly elected Mayor Gavin Newsom gave a green light to same-sex weddings earlier this month, the atmosphere was more muted on Monday as officials went back to requiring appointments for marriage licenses.

Only 56 appointments are granted per day, following a week in which officials said they had recorded the most marriages in recent San Francisco history. Frustrated applicants reported difficulty making appointments by telephone.

It remains unclear what legal weight the San Francisco marriage licenses would carry. The state's Department of Vital Records, which logs births, deaths and marriages, has said it will not accept the gay marriage certificates.

"This is deeply meaningful for us personally; we've been together for about 20 years now," said Carole Migden, a member of California's Board of Equalization, a tax collection agency, who got married on Friday. "It's a pivotal time in history."

Advocates for same-sex marriages have won initial legal skirmishes by blocking efforts to obtain an injunction against the marriages.