The issue of same-sex marriage is a controversial one in the US

Mayor of San Francisco Gavin Newsom issued the licences as a challenge to California's ban on same-sex marriage.

But city officials say they expect legal challenges.

No US state allows gay marriage, but the Supreme Court in Massachusetts made a landmark ruling earlier this month that such unions should be allowed.

The Massachusetts Legislature is grappling with a constitutional amendment that would block the court's ruling.

An outraged US conservative group said on Thursday it was taking legal action to thwart San Francisco's decision.

'Foolish' move

The group's leader Reverend Lou Sheldon told AFP: "This is a very foolish thing because the court will slap him [Mayor Newsom] down because he has violated the law.

"Newsom can't go against laws that are supported by most people. He has stepped totally out of his jurisdiction of authority politically and done a foolish thing."

The first same sex couple to get married under the mayor's new decree were lesbians Del Martin, 83, and Phyllis Lyon, 79, who have lived together for 51 years.

US Senate Majority Leader Bill Frist expressed concern that gay marriages would spread like "wildfire" and said the Senate would take up the issue this year.

Mr Frist gave no specifics but conservatives expect President George Bush to soon endorse a constitutional amendment banning gay marriage.

That bill has already been introduced in the US House of Representatives.