(Adds national poll, background, protester comments)
SAN FRANCISCO, June 16 (Reuters) - California begins marrying gay and lesbian couples on Monday afternoon in a step likely to challenge other states where laws define marriage as only between a man and a woman.
A state Supreme Court decision last month overturning a ban on gay marriage has already produced one major change: marriage licenses will no longer list bride and groom but rather Party A and Party B.
The landmark ruling goes into effect at the close of business on Monday, when San Francisco and West Hollywood, both known for a major gay population, will marry one couple each. Dozens of other couples will marry in a handful of smaller county offices open to all in the evening.
California is the second state to marry same-sex couples after Massachusetts, but it is the first ready to grant licenses to couples from any state. Gay marriage is rejected by 45 states, but New York will honor California unions.
"If marriages performed outside of New York are going to be recognized, I'm sure it won't be too long before New Yorkers will be able to be married in their own state. So already it is having an impact that crosses the impact to the Atlantic Coast," said Star Trek's Mr. Sulu -- actor George Takei -- who is marrying longtime partner Brad Altman.
"We are boldly going where no one has gone before," he said, jokingly echoing the opening of the TV series.
Many states and countries allow domestic partnerships though a relative few recognize gay marriage, including Massachusetts, Belgium, Canada, Spain and the Netherlands.
San Francisco Mayor Gavin Newsom, whose 2004 decision to marry gay and lesbian couples helped unleash the court battle ended last month, on Monday will marry Del Martin and Phyllis Lyon, an octogenarian pair who have been together more than 50 years and were the first married at City Hall four years ago.
Around the most populous U.S. state, with more than 36 million people, a few marriage offices will start ceremonies after 5 p.m. (8 p.m. EDT/0000 GMT) and work into the evening. Hundreds of volunteers have been deputized to marry couples in tents and on courthouse lawns.
Opponents aim to fight back in November when Californians will vote whether to change the state constitution to define marriage as between a man and a woman. Los Angeles' seven Catholic bishops on Monday said marriage "has a unique place in God's creation, joining a man and a woman."
Outside of the county clerk's office in Oakland, California, across the bay from San Francisco, a lone protester stood next to a camper covered in anti-gay marriage placards and criticized the state Supreme Court.
"I'd be of a mind to tar and feather them," said Ronald Brock, 69.
Acceptance of homosexual marriage has grown in the United States and abroad, although less than a third of Americans responding to a recent CBS poll say they should be legal. Over a third oppose gay marriage.
It was an issue in the 2004 U.S. presidential campaign but University of Southern California law professor David Cruz said gay marriage had not been a major topic this election year.
He predicted that the practicalities of married gay couples moving from California to other states would spark change.
"People's attitudes are already changing, and what will change public opinion in favor of same-sex marriages further is knowing same-sex couples and seeing them live their lives like other married couples," he said. (Additional reporting by Amanda Beck in Oakland and Syantani Chatterjee in Los Angeles) (Editing by Mary Milliken and Doina Chiacu)