Gov. Arnold Schwarzenegger on Thursday vetoed landmark legislation that would have made California the second state to allow same-sex couples to marry.

The Religious Freedom and Civil Marriage Protection Act, which squeaked through the legislature along strict party lines, would have recast the state's legal definition of marriage as a union between two people rather than between a man and a woman.

The Sept. 7 vote marked the first time that a state legislature approved a bill authorizing same-sex marriage without a court order. The Massachusetts legislature passed regulations permitting same-sex marriage, but only after the state's highest court ruled that it must.

"Clearly, the governor has failed the test of leadership and missed an historic opportunity to stand up for basic civil rights of all Californians," the bill's author, Assemblyman Mark Leno (D), said. "We will overcome this veto. Liberty and justice will prevail in spite of his lack of leadership."

In the run-up to the veto, which Schwarzenegger had signaled weeks ago, gay activists had run a series of television ads urging the action-hero-turned-politician to "be a hero" and sign the measure into law. Gay and lesbian activists said they plan to hold a series of protests in major cities across California on Friday.

In a statement, the Republican governor said that Californians had already voted on the subject in 2000 when they passed Proposition 22, also known as the Defense of Marriage Act, which held that marriage is between a man and a woman.

The constitutionality of that act has been challenged in court and will likely be decided by the U.S. Supreme Court, Schwarzenegger added. "This bill simply adds confusion to a constitutional issue," he said.

Schwarzenegger noted that California is a leader in "recognizing and respecting domestic partnerships and the equal rights of domestic partners." Indeed, more than 30,000 same-sex couples are registered in a state program. Schwarzenegger vowed to "vigorously defend and enforce these rights and as such will not support any rollback." This is significant because several groups are planning voter initiatives next year that would cut benefits to same-sex couples.

Schwarzenegger's popularity has sagged to its lowest point since he rolled to power on the back of a recall vote in 2003.

Californians are evenly split over the question of same-sex marriage. A Field Poll earlier this month showed 46 percent opposing and 46 percent approving such unions.

Arnold Schwarzenegger cited the approval of Proposition 22 in vetoing the legislature's bid to redefine marriage.