The lawsuit was brought by seven gay couples and three widowers, all of whom were married in Massachusetts after it became the first state in the country to legalize gay marriage in 2004.

In court documents filed Tuesday, the couples say the federal Defense of Marriage Act (DOMA) violates the equal protection clause of the U.S. Constitution because it denies them access to federal benefits given to other married couples, including pensions, health insurance and the ability to file joint tax returns. They argue that the law "eviscerates" the historic power of the states to establish criteria for marriage.

"DOMA marks a stark, and unique, departure from the respect and recognition the federal government has long afforded to State marital status determinations," lawyers for Gay & Lesbian Advocates & Defenders argue in a written response to the U.S. Department of Justice motion to dismiss the lawsuit.

In a court filing in September, Justice officials made it clear that the Obama administration thinks the law is discriminatory and should be repealed. But the department said it has an obligation to defend federal laws when they are challenged in court.

The law, enacted in 1996, was passed by Congress at a time when it appeared Hawaii would become the first state to legalize same-sex marriage. Opponents worried that other states would be forced to recognize such marriages.

In addition to Massachusetts, gay marriage is now legal in Vermont, Connecticut and Iowa. New Hampshire's law takes effect Jan. 1. Earlier this month, voters in Maine repealed a state law that would have allowed same-sex couples to wed.

A spokeswoman for the Justice Department had no immediate comment on the latest court filing by the same-sex couples.

In its written response to the lawsuit, filed in September, the Justice Department argued that there is no fundamental right to marriage-based federal benefits and says Congress is entitled to address issues of social reform on an "incremental" basis.

"Congress is therefore permitted to provide benefits only to those who have historically been permitted to marry, without extending the same benefit to those only recently permitted to do so," the government said.

The couples who brought the lawsuit are asking U.S. District Judge Joseph Tauro to reject the government's motion to dismiss the lawsuit and to find in their favor without a trial. Specifically, the group is asking for a ruling that the section of the law that excludes same-sex couples from federal marriage-based benefits is unconstitutional, as applied to the couples who brought the lawsuit.

Such a ruling would mean that those couples would be eligible for the benefits they have been denied. The ruling also would likely extend to other Massachusetts couples.

"If we won, then it would be unconstitutional to deny access to these programs to other married same-sex couples in Massachusetts," said Mary Bonauto, one of the attorneys representing the couples.

It would take a ruling from the U.S. Supreme Court or an act of Congress to strike down the law.

A bill to repeal the law was introduced in the U.S. House in September, but has little chance of making it to a vote this year.