US President George W Bush has spoken against gay marriage, as thousands of people from around the United States travel to San Francisco to make use of the city's issuing of marriage licences to same sex couples.

Since last week, Mayor of San Francisco Gavin Newsom has been allowing homosexual couples to marry because he says current law in California is discriminatory.

Judges have refused conservative demands to stop the weddings, and more than 2,500 licences have been issued to gays and lesbians so far.

But President Bush has said he is troubled by what he has seen, and he believes marriage should be defined as between a man and a woman.

He said his decision about whether to support a change to the constitution, banning gay unions, is being influenced by developments.