LOS ANGELES, May 27 (Xinhua) -- Two U.S. lawyers said here Wednesday that they had filed a lawsuit in federal court in hopes of overturning California's Proposition 8, which was approved by voters to ban same-sex marriage.

    Speaking at a press conference in downtown Los Angeles, Theodore Olson and David Boies said they filed the lawsuit on behalf of two homosexual couples in California.

    The two constitutional lawyers, who squared off in the legal battle that decided the 2000 U.S. presidential election, said they joined hands to seek an injunction to stay California's same-sex marriage ban while arguing it is a violation of the U.S. Constitution.

    Olson and Boies represented former President George Bush and his Democratic challenger Al Gore respectively in the Bush vs. Gore case, which decided Bush the winner of the tight-run 2000 presidential election.

    Wednesday's challenge in federal court came a day after the California Supreme Court upheld the validity of Proposition 8 under the state constitution. The proposition was approved by voters in last November's election but legally challenged by gay rights groups in the state high court.

    Olson and Boies filed the lawsuit in U.S. District Court in San Francisco against Californian governor Arnold Schwarzenegger and other state officials on behalf of two gay men and two lesbian women who wish to be married in California.

    The complaint argues that the voter-enacted proposition violates the U.S. constitutional guarantee of equal protection and due process.

    The case "is not about liberal or conservative, Democrat or Republican," said Olson at the press conference. "This case is about the equal rights guaranteed to every American under the United States Constitution."

    Boies said that while he and Olson are from different ends of the political spectrum, "we are fighting this case together because Proposition 8 clearly and fundamentally violates the freedoms guaranteed to all of us."