San Francisco - Voters in Florida and Arizona on Tuesday backed amendments to their state constitutions blocking gay marriage, while partial returns indicated that Californians would support a similar measure.

The constitutional amendments represented one of the major setbacks for centre-left voters, who saw Democratic nominee Barack Obama swept to a groundbreaking victory in the presidential election.

The closely watched referendum on gay marriage in California was winning with about 52.5 per cent support with about half of the vote counted. Earlier in the year, the California Supreme Court issued a ruling making such marriages legal under the state's constitution.

Voters in the US states of Michigan, South Dakota, Colorado and Massachusetts approved medical marijuana initiatives.

In Massachusetts, voters approved a measure to decriminalize the possession of small amounts of marijuana, making the offence punishable with a citation and a fine of 100 dollars.

In Michigan, voters approved the legalization of marijuana.

"Tonight's results represent a sea change," said Rob Kampia, executive director of the Marijuana Policy Project, which backed the Massachusetts and Michigan ballot proposals. "Voters have spectacularly rejected eight years of the most intense government war on marijuana."

In South Dakota, voters rejected an abortion ban, which allowed exceptions for cases of rape, incest and severe risk to the life of the mother. Voters in South Dakota rejected an even stricter proposal two years ago that did not include exceptions for rape and incest.

Colorado failed in a far-reaching constitutional amendment that would have defined human life as beginning at conception. The adoption of this referendum would have made any abortion illegal while extending human and property rights to foetuses.

In another major ballot initiative, 58 per cent of voters in the state of Washington approved a suicide measure that allows terminally ill people with a medical estimate of less than six months to live to ask doctors for lethal drugs that they could self-administer.

California voters approved a proposition that would require farmers to provide space for egg-laying hens, veal calves and pregnant sows to move around in their pens.