The U.S. Supreme Court this morning struck down part of a federal law that restricts the definition of marriage to opposite-sex couples in a major victory for the gay rights movement.

The ruling, on a 5-4 vote, means that legally married gay men and women are entitled to claim the same federal benefits that are available to opposite-sex married couples.

The court was due to decide within minutes a second case concerning a California law that bans same-sex marriage in the state.

The federal case concerns the Defense of Marriage Act (DOMA), which limits the definition of marriage as between a man and a woman for the purposes of federal benefits. It permits benefits such as Social Security survivor payments and federal tax deductions only for married, opposite-sex couples, not for legally married same-sex couples.

President Bill Clinton signed DOMA into law in 1996 after it passed Congress with only 81 of 535 lawmakers opposing it. Clinton, a Democrat, said earlier this year that times had changed since then and called for the law to be overturned.

The DOMA case before the court focuses on whether Edith Windsor of New York, who was married to a woman, should get the federal estate tax deduction available to heterosexuals when their spouses pass away.

Windsor's marriage to Thea Spyer was recognized under New York law, but not under the federal law. When Spyer died in 2009, Windsor was forced to pay federal estate taxes because the federal government would not recognize her marriage. She sued the government, seeking a $363,000 tax refund.

President Barack Obama's administration initially defended the law but in 2011 Attorney General Eric Holder said the law was unconstitutional. The administration asked the Supreme Court to strike the law down. During oral arguments in March, it appeared the justices may be poised to do so. Obama is the first sitting president to support gay marriage.

The California case involves a state ballot initiative, called Proposition 8, which was approved by voters in 2008 just months after the state's high court said gay marriages could proceed.

A federal judge struck the law down in 2010, saying it violated the U.S. Constitution's guarantee of equal protection. Proponents of the ban appealed, setting in motion the journey to the Supreme Court.

The two couples who filed suit were Kristin Perry and Sandra Stier, from Berkeley, and Jeffrey Zarrillo and Paul Katami, who live in Burbank.

The cases are United States v. Windsor, U.S. Supreme Court, No. 12-307 and Hollingsworth v. Perry, U.S. Supreme Court, No. 12-144.

Advocates for and against gay marriage waited anxiously outside the Supreme Court Wednesday morning for potentially historic decisions.

Pro gay marriage activists sang Unitarian church songs and "God Bless America. Signs reading "#gay," "it's about taxation not representation," and "It's about time" dotted the steps of the Supreme Court, with supporters of same sex marriage seeming to outnumber opponents.

But one supporter of traditional marriage said he believes "taxpayers don't have to pay for the benefits of people with sexual preferences."

"I don't think this decision would change anything in reality," said Larry Cirignano of New Jersey, who said his stances comes from religious and political beliefs. "We will continue to have this conversation over a long time. Thirty eight states still say same sex marriage is not legal."

Bryan Leon married his husband Jeremy in Washington, D.C. in 2010, and arrived to get in line around 5 in the morning.

"We're federal employees, so this decision can really make a difference for us," said Leon, who works at the Office of Management and budget. He was speaking of the Defense Of Marriage Act, which blocks legally married gays from receiving federal benefits.

David Baker, originally from Salt Lake City, Utah, describes himself as a gay Mormon, and has previously worked with Scouts for Equality and on LGBT issues in his church.

"If I can't marry my boyfriend, I'll marry your daughter!" Baker read from the cardboard poster he held up. "If I'm going to continue to be treated as a second class citizen, I might as well... it's amazing that in the hands of nine people stands the balance of my rights."

Earlier in the week, the court issued rulings on the Voting Rights Act and affirmative action in higher education.

Reuters and Marina Villeneuve, Los Angeles Times