By Branwen Jeffreys BBC, Washington

Couples have been hurrying to wed

The president said he was troubled by the weddings, and would only support a law backing traditional unions.

More than 2,500 marriage licences have been issued to gays and lesbians.

The city's mayor took the initiative, contrary to California state law, because he said that current legislation was discriminatory.

Since Mayor Gavin Newsom allowed same-sex couples to marry, gays and lesbians have travelled from across the US to take part in what they see as a historic moment for gay rights.

I'm troubled by what I've seen George W Bush

But speaking for the first time since the weddings began last week, the president made clear his personal opposition.

"I strongly believe that marriage should be defined as between a man and a woman," said Mr Bush.

"I am troubled by activist judges who are defining marriage. I have watched carefully what's happened in San Francisco where licences were being issued, even though the law states otherwise."

When asked whether he was ready to take action, he said that he was "just watching very closely".

"I'm troubled by what I've seen," he added. "People need to be involved with this decision. Marriage ought to be defined by the people not by the courts, and I'm watching it carefully."

There are already two legal challenges underway to the decision of San Francisco's mayor to issue the wedding licences, and the legal status of gay marriages is being contested elsewhere.

In Massachusetts, a court recently said it was "unconstitutional" to prevent gay and lesbian couples from marrying.