Los Angeles ? A federal judge in Boston on Thursday ruled a key element of the Defense of Marriage Act unconstitutional, saying it violates the Equal Protection Clause and interferes with a state’s right to define marriage.

Gay rights advocates hailed the rulings as important steps toward striking down the law, which they say discriminates against their sexual orientation. Opponents denounced it as judicial activism.

The 1996 law prohibits the federal government from recognizing same-sex marriages, depriving gay spouses of federal benefits such as health care and tax breaks that their heterosexual counterparts receive.

In two separate cases, plaintiffs argued that the law unfairly discriminated against gay spouses, even in states where their marriages are legal.

The rulings by U.S. District Judge Joseph Tauro deal only with the law as it affects Massachusetts residents. But they could eventually help secure federal benefits for other gay spouses in states where such marriages are legal, as well as for the estimated 16,000 gay couples who wed in California in 2008.

Massachusetts Attorney General Martha Coakley had argued in a lawsuit that the law, known as DOMA, was forcing her state to discriminate against its own citizens.

Tauro agreed, saying the act “plainly encroaches” on a state’s right to define marriage.

“Today’s landmark decision is an important step toward achieving equality for all married couples in Massachusetts and assuring that all of our citizens enjoy the same rights and protections under our Constitution,” Coakley said. “It is unconstitutional for the federal government to discriminate, as it does because of DOMA’s restrictive definition of marriage.”